W5...” #2... 1‘. S. I file 2:: N. a. . . ‘. a av. 4:35». .u 3..“ , wflw - I. .?..mu~ww.n 14:3. c lanai: , , . 10 A: 12.11: 4... ‘ v1.1.1.3: .LIBRARY Michigan State University This is to certify that the dissertation entitled PRODUCTION AND PROPERTIES _OF BIOMAX® Doctoral MICROCELLULAR FOAMS presented by Napawan Kositruangchai has been accepted towards fulfillment of the requirements for the degree in School of Packaging Major Professor’s Signature Ska/W Date MSU is an affinnative—action, equal-opportunity employer --a-—. -o—u—o-u-o-v---—--'-- PLACE IN RETURN BOX to remove this checkout from your record. TO AVOID FINES return on or before date due. MAY BE RECALLED with earlier due date if requested. DATE DUE DATE DUE PRODUCTION AND PROPERTIES OF BIOMAX® MICROCELLULAR FOAMS By Napawan Kositruangchai A DISSERTATION Submitted to Michigan State University "In partial fulfillment of the requirements For the degree of DOCTOR OF PHILOSOPHY School of Packaging 2007 ABSTRACT PRODUCTION AND PROPERTIES OF BIOMAX® MICROCELLULAR FOAMS BY NAPAWAN KOSITRUANGCHAI Awareness and concern about the environment, especially municipal solid waste (MSW), is significant in the US. The weight of plastic in MSW generated was about 28.9 million tons and the weight recovered was only 1.65 million tons, 5.7%, in 2005. For MSW in the US, plastics are one of the main concerns, occupying about 9 to 12 % of the landfills by volume. ® Biomax is a biodegradable plastic, one alternative to conventional polymers from ® an environmental viewpoint. The cost of Biomax is about $1.80 per 1b which is expensive compared to other biodegradable polymers such as poly(lactic acid) (85¢/lb). This has significantly limited its usefulness, so microcellular foam is one way to reduce the material used per item, and thereby to reduce cost. Microcellular foam is characterized by cell sizes in the range of 0.1 to 10 um, and has a cell density in the range of 109-1015 cell/cm3. The unique features of microcellular foam are fine cell size, high cell density, and use of inorganic blowing agents. ® Biomax microcellular foams were successfully developed, yielding closed cell foams. These foams changed from translucent and brownish in the unfoamed Biomax® samples to white and Opaque, with smooth surfaces. In this study, the effects of foaming temperature and foaming time on size, density, average cell size, cell population density, morphology, tensile strength, percent elongation at break, and tensile modulus of Biomax® microcellular foams were studied. The experimental results for tensile strength and tensile modulus were compared with the simple rule of mixtures and with Moore’s empirical square power law, respectively. The results were analyzed using one way ANOVA with 95% confidence interval. With increasing foaming temperature, the polymer chain stiffness decreased, causing the viscosity and surface tension of polymer to decrease (less retarding) so the diffusion rate of C02 gas from the polymer to the cells was higher, leading to increased cell expansion, increased size of the foam, larger cell sizes, and decreased cell wall thickness and density. With longer foaming time, C02 gas had a longer time to diffuse into the cells, so the cells expanded more, leading to larger size, and reduction in density and wall thickness. Tensile strength and modulus decreased with increasing foaming temperature and foaming time. Experimental results for tensile and tensile modulus at different foaming temperatures showed a good fit with the model, and tensile modulus at different foaming times showed a good fit with the model. After one year, foamed samples became slightly stiffer. Tensile strength increased slightly, but percent elongation decreased. The thickness of samples affected the uniformity of the foams. At higher thicknesses, the inside of the foam needed a longer time for gas to diffuse and higher foaming temperature because of the temperature gradient. The outside surface of foams had higher temperature so larger cell sizes than the inside samples. At the same processing conditions, the thicker samples had higher density, smaller cell sizes, and thicker cell walls than did the thinner samples. Acknowledgements I would like to express my sincere appreciations to my advisor, Professor Susan Selke, for her invaluable guidance, encouragements, supporting, understanding throughout my program. Not only did she lead me through this program, but also in various aspects of my personal development. I am also grateful to Dr. Laurent Matuana, one of my committee members, for his valuable help; especially the foaming equipment, his advice, and his encouragement. I also would like to specially thank to Dr. Bruce Harte for his understanding, supporting, advice, and encouragement and serving as my committee member. I also would like to acknowledge Dr. Rafael Auras for serving as my dissertation committee and for his invaluable comments and support. I also would like to express my thanks to the Royal Thai Government for the Scholarship and support of my study at Michigan State University. Also I would like to acknowledge the financial support provided by the Center for Food and Pharmaceutical Packaging Research (CFPPR) in the past two years. I also would like to thank DuPont for providing the material for this study. I would like to thank Bhavesh Shah and Karana Colbum for their help, discussion, friendship, and supporting, and Sungwom Ngudgratoke for his statistical help. I also would like to acknowledgement to Mike Rich and Brian Brook for teaching me to operate the mini-extruder with injection molding. Thanks also go to Carol Flegle and Ewa Danielewicz for valuable help with the seaming electron microscope (SEM). iv Thanks also go to all faculty, staff, Thai students: Nueng, Tum, Hall, Ploy, Turk, Varee, Pim, Oh, Mod, and Jaew, friends in the School of packaging: Yu Tai, Pankaj, Isinay, Koushik, etc. I also have special thank to Daniel Yalda for your support, understanding, patient and valuable advise. Last but not least, I would like to thank my parents and my family for their love and support. Table of Contents List of Tables .......................................................................... List of Figures ........................................................................ Chapter 1: Chapter 2: Chapter 3: Introduction Objectives ........................................................... References .......................................................... Literature Review .................................................. Degradable polymers. Biodegradable polymers Starch and derivatives .................................... Polylactic acid (PLA) ....................................... Polyhydroxyalkanoates (PHAs) ......................... Ecoflex .................................................... Poly(c-caprolactone) (PCL) ............................... Biomax® .................................................... Polymer foams ...................................................... Advantages of microcellular foams over conventional foams Microcellular foams ............................................... Semi-continuous microcellular process ........................ Continuous microcellular process ............................... Blowing agents ..................................................... Carbon dioxide and its interaction with polymers ............ Solubility of C02 in polymers ................................... Effect of processing conditions on microcellular foams. . Effect of crystallinity on microcellular foams ................. Properties of polymer microcellular foams .................... References ......................................................... Materials, Methods, Sample Preparation, Sorption Test, and Characterization of Samples and Foamed Samples ............................................................ Materials and methods .......................................... Materials ........................................................... Methods ............................................................ Characterization of Biomax® pellets ........................... Therrnogravimetric analyzer (TGA) .................. Differential Scanning Calorimetry (DSC) ........... Preparing Biomax® specimens ................................. Twin-screw extruder .................................... Twin-screw extruder conditions ....................... Compression molding .................................... vi xiiii OA— 10 ll 12 14 15 18 18 19 21 22 23 29 30 31 32 33 35 37 39 42 51 52 52 52 52 52 54 55 55 56 56 Chapter 4: Compression molding conditions ...................... Mini twin-screw extruder and injection molding. . .. Mini twin—screw extruder conditions .................. Foaming process: batch process .................................. Comparison of processing conditions .................. Twin-screw extruder with compression molding (Sample I) ................................................. Mini twin-screw extruder with injection molding (Sample 11) ................................................ Comparison of foams from the 2 methods. . . . . The saturation time of C02 uptake of Biomax® specimens. Materials .................................................... Methods .................................................... Selection of sorption conditions. . . . . . . . . . . . . . . . . Characterization of foams ........................................ Dimensions ........................................ Density ..................................................... Void fraction .............................................. Morphology analysis .................................... Determination of cell density and average cell size Testing .............................................................. Mechanical properties ................................... References ......................................................... The Effect of Foaming Temperature on Characteristics and Tensile Properties of Biomax® Microcellular Foams ............................................................... Materials and methods ............................................ Materials ................................................... Preparing samples ......................................... Batch process .............................................. Statistical analysis ......................................... Results and discussion ............................................. Effect of foaming temperature on foamed dimensions Effect of foaming temperature on foamed densities, relative density and void fraction of Biomax® microcellular foams ....................................... Effect of foaming temperature on cell population density and average cell size of Biomax® microcellular foams ....................................... Effect of foaming temperature on morphology of Biomax® microcellular foams ........................... Effect of foaming temperature on tensile strength of Biomax® microcellular foams ...................... vii 57 57 58 60 60 60 62 63 66 66 66 66 69 69 69 69 70 72 72 72 73 74 77 77 77 77 77 78 78 80 81 83 86 Chapter 5: Chapter 6: Relationship between relative tensile strength and relative density ........................................... Effect of foaming temperature on percent elongation at break of Biomax® microcellular foams Effect of foaming temperature on tensile modulus of Biomax® microcellular foams ....................... Relationship between relative tensile modulus and relative density ............................................. Conclusions ......................................................... References .......................................................... The Effect of Foaming Time on Characteristics and Tensile Properties of Biomax® Microcellular Foams...... Materials and methods ............................................ Materials ................................................... Preparing samples ....................................... Batch process ............................................. Results and discussion ............................................. Effect of foaming time on foamed dimensions ....... Effect of foaming time on foamed density of unfoamed Biomax® and foams ...................... Effect of foaming time on cell population density and average cell size of Biomax® microcellular foams Effect of foaming time on morphology of Biomax® microcellular foams ..................................... Effect of foaming time on tensile strength of Biomax® microcellular foams ......................... Relationship between relative tensile strength and relative density ........................................... Effect of foaming time on percent elongation at break of Biomax® microcellular foams ............... Effect of foaming time on tensile modulus of Biomax® microcellular foams ........................ Relationship between relative tensile modulus and relative density ............................................. Relationship between average cell sizes and densities At different foaming temperatures and foaming time Conclusions ......................................................... References .......................................................... Effect of Thickness and Aging on Tensile Properties of Unfoamed and F oamed Biomax® ..................... Materials and Methods ....................................... Materials ............................................... viii 88 90 92 93 96 98 100 100 100 100 100 101 101 103 104 106 108 110 111 113 114 116 118 119 120 122 122 Preparing samples ....................................... Batch process .............................................. Results and discussion .......................................... Effect of foaming temperatures and times on foamed dimensions ..................................... Effect of foaming temperature and foaming time on foamed densities ...................................... Effect of foaming temperature and foaming time on cell population density and average cell size of foams .................................................. Effect of foaming temperature and foaming time on foamed morphology ................................... Relationship between average cell sizes and densities ...... The effect of thickness of dumbbell shape (1.59 mm) and rectangular shape (2.12 mm) on size, density, cell population density, and average cell size of Biomax® microcellular foams ............................................... Materials and methods ............................................ Results and discussion ............................................ Effect of sample thickness on foamed dimensions. Effect of thickness on foamed densities .............. Effect of thickness on cell population density of foams ...................................................... Effect of thickness on average cell size of foams. The aging effect on tensile properties ........................... Materials and methods .................................. Results and discussion .................................. Conclusions ....................................................... References .......................................................... Chapter 7: Conclusions and Future Work Conclusions ...................................................... Future work ..................................................... Appendix ............................................................................ ix 122 122 123 123 125 128 130 139 141 141 141 141 143 144 145 147 147 147 151 152 153 153 156 158 Tables Chapter 2 Chapter 3 Chapter 4 List of Tables Table 2.1: Properties of Biomax® 4026 and PET ............ Table 3.1 :The glass transition temperature, melting temperature, and heat of fusion of Biomax® pellets ......... Table 3.2: Results of weight gain at the different saturation times in the C02 high-pressure chamber ....................... Table 4.1: Dimensions of unfoamed Biomax® and foamed at different foaming temperatures and at 10 sec foaming time; a) length, b) width, and c) thickness ........................... Table 4.2: Density, relative density, and void fraction of unfoamed Biomax® and foamed at different foaming temperatures with 10 sec foaming time ........................ Table 4.3: Cell population density and average cell size of Biomax® microcellular foams at the different foaming Temperatures with foaming time of 10 sec ..................... Table 4.4: Tensile strength, specific tensile strength, density, and relative density of unfoamed Biomax® and foams at different foaming temperatures at 10 sec foaming time ...... Table 4.5: Tensile strength, relative tensile strength, and relative density of unfoamed Biomax® and foamed samples at different foaming temperatures ............................... Table 4.6: Percent elongation at break of unfoamed Biomax® and foams at different foaming temperature at foaming time 10 sec ............................................. Table 4.7 Tensile modulus, relative tensile modulus, and relative density of unfoamed Biomax® and foams as a function of foaming temperatures at 10 sec foaming time. Table 4.8 Tensile modulus, relative tensile modulus, and relative density of unfoamed Biomax® and foams as a function of the foaming temperature at foaming time 10 see Page 20 54 67 79 81 82 87 89 91 92 94 Chapter 5: Chapter 6: Table 5.1 Dimensions of unfoamed Biomax®and foams at different foaming times at 120°C foaming temperature, a) length, b) width, and c) thickness ............................ Table 5.2 Density of unfoamed Biomax®and foams at different foaming times at 120°C foaming temperature Table 5.3 Cell population density and average cell size of Biomax® foams at different foaming times with foaming temperature 120°C ................................................ Table 5.4 Tensile strength and specific tensile strength of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C ................................. Table 5.5 Tensile strength, relative tensile strength, and relative density of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C. .. Table 5.6 Percent elongation at break of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C ............................................................... Table 5.7 Tensile modulus, specific tensile modulus density, and relative density of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C ............................................................... Table 5.8 Modulus, relative modulus and relative densities of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C ................................... Table 5.9 Density and the average cell size at different foaming temperatures and times ................................. Table 6.1Dimensions of unfoamed Biomax® and foams at different foaming temperatures and times ..................... Table 6.2 Results of density, relative density, and void fraction of unfoamed Biomax®, and foams at different foaming temperatures and times ............................... Table 6.3 Comparison of percent decrease of density at foaming times (5, 10 sec) at different foaming temperatures xi 102 103 105 109 111 112 114 115 116 123 126 126 Table 6.4 Comparison of percent decrease of density at Foaming temperatures (120, 130, and 140°C) at different foaming times ..................................................... Table 6.5 Results of cell population density and average cell size of unfoamed Biomax® and foams at different foaming temperatures and times .......................................... Table 6.6 Comparison of percent increase of average cell size at foaming times (5, 10 sec) at different foaming temperatures ........................................... Table 6.7 Comparison of percent increase of density at foaming temperatures (120, 130, and 140°C) at different foaming times ........................................ Table 6.8 Densities and average cell sizes at different foaming temperatures and times .............................. Table 6.9 Comparison of dimensions between thinner dumbbell and thicker rectangular shape, at different temperatures a) length, b) width, and c) thickness ............ Table 6.10 Comparison of density between thinner dumbbell and thicker rectangular shape at different foaming temperatures at 10 sec foaming time ........................... Table 6.11 Compared cell population density between thinner dumbbell and thicker rectangular shape at different foaming temperatures at 10 sec foaming time ........................... Table 6.12 Comparison of average cell size between thinner dumbbell and thicker rectangular shape at the different foaming temperatures at 10 sec foaming time .................. Table 6.13 The melting temperature results of unfoamed and foamed Biomax® at 2 weeks and at lyear storage time after foaming .............................................................. Table 6.14 The heat of fusion results of unfoamed Biomax® and foamed at 2 weeks and at lyear storage time after foaming .............................................................. xii 127 128 129 130 139 142 143 144 146 147 148 Table 6.15 Tensile strength of unfoamed Biomax® and Foams at 2 weeks and at 1 year storage time after foaming ............................................................ 150 Table 6.16 Percent elongation at break of unfoamed Biomax® and foams at 2 weeks and at lyear storage time after foaming .................................................... 150 Table 6.17 Tensile modulus of unfoamed Biomax® and foams at 2 weeks and at 1 year storage time after foaming ......................................................... 150 xiii Figures Chapter 2 Chapter 3 List of Figures Figure 2.1: Total waste generation and MSW generated per person in the US from 1960-2005 ................................. Figure 2.2:Total MSW generation in 2005 in the US .......... Figure 2.3 Molecular structure of lactic acid ................... Figure 2.4: Structure of 3HB ...................................... Figure 2.5: Structure of Ecoflex® ................................. Figure 2.6: Semi-continuous microcellular process ........... Figure 3.1: TGA 2950 from TA Instruments .................. Figure 3.2: TGA results of Biomax® pellet .................... Figure 3.3: DSC Q 100 from TA Instruments ................ Figure 3.4: DSC result of Tg and Tm ........................... Figure 3.5: Compression molding .............................. Figure 3.6: Samples compression molding, and cut samples Figure 3.7: Mini-extruder with injection molding system with its molds; a) twin screw extruder, b) injection molding system, c) molds, and (1) samples .................. Figure 3.8: Foamed samples of sample I at a) 100, b) 105, c) 110, and d) 120 °C foaming temperatures at the foaming times of unfoamed, 5 sec, 10 sec, 15 sec and 20 see, from left to right respectively .......................................... Figure 3.9: Foamed samples of sample 11 at 100 °C at the foaming time of unfoamed, 5, 10, 20, and 30 sec, left to right respectively, and b) 120 °C at unfoamed, 5, and 15 sec foaming time from left to right ................................. Figure 3.10: a) Unfoamed and foamed sample 1, b) Unfoamed and foamed sample 11, c) Unfoamed and foamed samples of ample II and II from left to right ................................. xiv Page 10 14 17 18 30 53 53 54 55 56 57 59 62 63 65 Chapter 4 Figure 3.11: CO; uptake in the Biomax® matrix .............. Figure 3.12: Samples of unfoamed Biomax® and foams at 1, 2, 3, 4, 5, and 6 days of saturated time from left to right. Figure 3.13 Emscope Sputter Coater a) Emscope, b) sample holder, and c) samples .............................................. Figure 3.14: The Scanning Electron Microscope (SEM): J SM-6400 with a LaB6 emitter .................................. Figure 4.1: Unfoamed Biomax® and foamed samples at 100, 110 120, and 130°C foaming temperatures (from left to right) at 10 sec foaming time ................................. Figure 4.2 Foamed density at different foaming temperatures at foaming time of 10 sec .......................................... Figure 4.3 Cell population density of Biomax® microcellular foams at different foaming temperatures at 10 sec foaming time .................................................................. Figure 4.4 Average cell size of Biomax® microcellular foams at different foaming temperatures at foaming time 10 sec. .. Figure 4.5 SEM micrographs of Biomax® microcellular foams at 10 sec foaming time at a magnification of 1000, at a) 100°C, b) 110 °C, 0) 120°C, d) 130°C, and e) 140°C.. Figure 4.6 Stress-strain curves of unfoamed Biomax® and foamed samples at different foaming temperatures at 10 sec foaming time ...................................................... Figure 4.7 Tensile strength and specific tensile strength of unfoamed and foamed Biomax® at different foaming temperatures and a foaming time of 10 sec .................... Figure 4.8 Relative tensile strength as a function of relative density of Biomax® foams at different foaming temperatures at 10 sec foaming time ........................... Figure 4.9 Percent elongation at break of unfoamed Biomax®and foamed samples at different foaming Temperatures at 10 sec foaming time ........................... XV 68 68 70 71 80 81 82 83 85 87 88 90 91 Chapter 5: Figure 4.10 Tensile modulus and specific tensile modulus of unfoamed Biomax® and foams as a function of foaming temperature at foaming time 10 sec ............................ Figure 4.11 Relative tensile modulus as a function of relative foam density of unfoamed Biomax® and foams at different foaming temperatures at 10 sec foaming time... Figure 5.1 Unfoamed Biomax® and foamed samples at foaming temperature 120°C at 10, 20, and 30 sec fiom left to right ...................................................... Figure 5.2 Density of Biomax® foams at different foaming times with foaming temperature120°C ....................... Figure 5.3 Cell population density of Biomax® foams at the different foaming times with foaming temperature at 120°C. Figure 5.4 Average cell size of Biomax® foams at different foaming times with 120°C foaming temperature .............. Figure 5.5 SEM micrographs of Biomax® microcellular foam at the different foaming times for 120°C foaming temperature at magnification of 1000, a) 10 see, b) 20 sec, and c) 30 sec. Figure 5.6 Stress-strain curves of Biomax® and foams at different foaming times at 120°C foaming temperature. . .. Figure 5.7 Tensile strength and specific tensile strength of unfoamed Biomax®and foams at different foaming times at foaming temperature 120°C .................................. Figure 5.8 Relative tensile strength as a function of relative density ........................................................... Figure 5.9 Percent elongation at break of Biomax® foams at different foaming times at foaming temperature 120°C. .. Figure 5.10 Tensile modulus and specific tensile modulus of unfoamed Biomax®and foams at different foaming times at 120°C foaming temperature ................................... Figure 5.11 Relative tensile modulus as a function of the relative density .................................................... xvi 93 95 103 104 105 106 107 109 110 111 113 114 115 Chapter 6 Figure 5.12 Average cell size as a function of density of Biomax® foams at different foaming temperatures and foaming times ...................................................... Figure 6.1 Effect of foaming temperatures on size of Biomax® microcellular foams at different foaming times: a) 10 sec and b) 5 sec ......................................... Figure 6.2 Effect of foaming times on the size of Biomax® microcellular foams at the different foaming temperatures a) 120°C at 10 and 15 see, b) 130°C at 5 and 10 sec, and c) 140°C at 5 and 10 sec ........................................ Figure 6.3 Density of Biomax® microcellular foams at different foaming temperatures and times .................... Figure 6.4 Cell population density of Biomax® microcellular foams at be different foaming temperatures and times ...... Figure 6.5 Average cell sizes of Biomax® microcellular and foams at different foaming temperatures and times ........... Figure 6.6 SEM images of 120°C at 10 sec at foaming time, magnificationlOOO ................................................. Figure 6.7 SEM images of 120°C at 15 sec at foaming time, magnificationlOOO ................................................. Figure 6.8 SEM images of 130°C at 5 sec at foaming time, magnificationlOOO ................................................. Figure 6.9 SEM images of 130°C at 10 sec at foaming time, magnificationlOOO ................................................. Figure 6.10 SEM images of 140°C at 5 sec at foaming time, magnificationlOOO ................................................. Figure 6.11 SEM images of 140°C at 10 sec at foaming time, magnification 1 000 ................................................. Figure 6.12 SEM images of 150°C at 5 sec at foaming time, magnificationlOOO ................................................. xvii 117 124 125 127 129 130 132 133 134 135 136 137 138 Figure 6.13 Average cell sizes as a function of the relative densities .......................................................... Figure 6.14 Comparison of density between thinner dumbbell and thicker rectangular shape at different foaming temperatures at 10 sec foaming time ............................ Figure 6.15 Compared cell density between thinner dumbbell and thicker rectangular shape at different foaming temperatures at 10 sec foaming time ........................... Figure 6.16 Comparison of average cell size between thinner dumbbell and thicker rectangular shape at different foaming temperatures at 10 sec foaming time ............................ Figure 6.17 Stress-strain curves of unfoamed Biomax and Foams between 2 weeks and lyear storage time ............... xviii 140 144 145 146 149 Chapter 1 Introduction Awareness and concern about the environment, especially municipal solid waste (MSW), arose in the mid 19803 in the US. The basic problem was that in some areas of the US, there was a significant decrease in the availability of landfill space, more in major cities than in smaller towns or rural areas [1]. The communities in the US are taking an integrated approach to disposing of municipal waste. Almost every community has some type of recycling program and encourages citizens to practice the 3R5 (reduce, reuse, and recycle) to reduce the amount of waste generated. A very important component of minimizing waste is source reduction, which can be achieved by redesigning products to be lighter and using fewer materials of manufacture [2]. In the US, in 2005, the US. Environmental Protection Agency (EPA) reported that about 245.7 million tons (about 4.5 pounds of waste per person per day) of municipal solid waste (MSW) was generated, and 133.3 million tons went to landfill. For MSW in the US, plastics are one of the major concerns due to the volume of plastics in which it occupies about 9 to 12 % of the landfills by volume. The weight of plastic in MSW generated was about 28.9 million tons and the weight recovered was only 1.65 million tons, 5.7%, in 2005 [3]. Some plastic waste has a high degree of contamination, especially from food, and therefore it is difficult to recycle or reuse [4]. One focus of attempts to deal with the problem is to look for ways to minimize MSW generation [1]. Today’s polymeric materials are designed with consideration for their ultimate disposability or recyclability. Biodegradable plastics could alternatively replace the conventional plastics and this waste could end up in composting facilities instead of landfills or incinerators. ASTM D 6400-04 defines “Biodegradable plastics are a degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi, and algae and results finally in production of carbon dioxide and water” [5]. The biodegradable plastics are interesting from a sustainability view for composting containers such as fast food containers, which could. reduce the volume of these one-time use containers in the landfills. Biomax® is a material from DuPont, and is a hydro/biodegradable polyester. The material is based on polyethylene terephthalate (PET) [6] and has up to three different comonomers for integration into the polymer. The comonomers create weak points in the polymer chains, which make them susceptible to degradation through hydrolysis. When the large polymer molecules have been broken down into smaller molecules by the action of moisture, naturally occurring microbes consume the small molecules, converting them into carbon dioxide and water. Biomax® can be manufactured in conventional PET equipment, making its processing only a little more expensive than PET [6]. Biomax® 4026 can be used as a film or coating in disposable food service packaging, such as bowls, plates, cups, sandwich wraps and clamshell sandwich containers, agricultural films, seed mats, plant pots, bags that cover ripening fruit, thermoforrned packaging; blown bottles; and injection-molded objects. Biomax® 4026 Polyester Resin complies with Food and Drug Administration Food Contact Notification (FCN) #355, which describes polymers that may be used as a coating or film in food-contact articles [7]. The cost of Biomax® is about $1.80 per lb [8] which is expensive compared to other biodegradable polymers such as poly(lactic acid) (85¢/lb) [9]. This has significantly limited its usefulness, so microcellular foam is one way to reduce the material used per item, and thereby to reduce cost. Microcellular foam is characterized by a cell population density in the range of 109-1015 cell/cm3, and has cell sizes in the range Of 0.1 to 10 um, differentiating it from conventional cellular plastics with cells of about 100 um and above [10-12]. The unique features of microcellular foam are fine cell size, high cell population density, and use of inorganic blowing agents. In many cases, microcellular plastics show high impact strength, high toughness, high stiffness-to-weight ratios, high fatigue lives, high thermal stability, low dielectric constants and low thermal conductivity [13]. But microcellular foams need very specific processing conditions, especially high saturation pressure and high pressure drop rate [10]. Biomax® has up to three comonomers linked to a PET backbone; consequently, Biomax® has lower crystallinity. Therefore, C02 gas as a blowing agent can more easily dissolve and diffuse into a Biomax® matrix, compared to PET, so Biomax® has potential to be used in microcellular foam. In the batch process of microcellular foaming, the expansion process consists of 3 main steps: nucleation, bubble growth and stabilization [14]. After samples have been supersaturated with blowing agent, the first step is bubble nucleation. Once nucleated, the bubble continues to grow as the blowing agent diffuses into it. This growth will continue until the bubble stabilizes or ruptures. The foam density decreases as the available blowing agent molecules diffuse into the cells. The growth rate Of the cells is limited by the diffusion rate and the stiffness of the viscoelastic polymer/gas solution. In general, cell growth is affected primarily by the time allowed for the cells to grow, the temperature of the system, the state of supersaturation, the pressure or stress applied to the polymer matrix, and the viscoelastic properties of the polymer/ gas solution. As cell expansion increases, the cell wall thickness decreases and as a result, the rate of gas diffusion between cells increases, so density decreases and void fraction increases more rapidly. ® Biomax microcellular foams are one alternative to conventional polymers. They combine use of a hydro/biodegradable polyester to be environmentally friendly, and at the same time microcellular foaming to reduce the amount of polymer used and therefore the cost. Scope of this study Objectives The focus on this study was to investigate Biomax® microcellular foams produced by a batch process using CO2 as a blowing agent. The specific Objectives were: 1. To find a suitable method to prepare Biomax® specimens for batch processing, choosing between twin-screw extruder and then compression molding, and mini extruder with injection molding. 2. To study the saturation time Of CO2 uptake of Biomax® microcellular foams. 3. To investigate the effect of foaming temperature and time on size, density, cell population density, average cell size, morphology, and mechanical properties of Biomax® microcellular foams. 4. To study the effects of thickness on size, density, cell population density, and ® average cell size of Biomax microcellular foams at different foaming temperatures. 5. To study the effect of aging on tensile strength, elongation at break, and tensile modulus of Biomax® microcellular foams. References 1. 10. 11. 12. 13. Hernandez, R., Selke, S. E., and Culter, J. D., Plastic Packaging: Properties, Processing, Applications, and Regulations, Hanser Publishers, Munich (2000). Environmental Literacy Council, “Municipal Solid Waste”, http://wwwcnvirolitcracyorgfiirticle.php/204.html (accessed on April 11, 2007). . US. Environmental Protection Agency, “Municipal Solid Waste in the United States: 2005 Facts and Figures” http://www.cpagm/garbagc/pubs/cx-sumOSfipdf, (accessed on April, 2007). Weber, C. 1., (ed), Biobased Packaging Materials for The Food Industry, Status and Perspectives: EUD Directorate 12 (2000). . ASTM International Standards Worldwide, ASTM D 6400-04 “ Standard Specification for Compostable Plastics” (2006). Dupont, “DuPont Biomax Resins” http://dupont.com/corp/news/releases/mediat’pdtiv’biomaxngdf (accessed on April, 2007) DuPont Packaging and Industry Polymers, Product information of Biomax® 4026 Hydro/biodegradable Polyester Resin, v 5, Feb (2004). Pamm, R. C ., Dupont, Personal Communication (via e-mail), Feb 2005. Schut, J. H., Extruding Biopolymers Packaging Reaps Cost Benefit of Going‘Green’, Plastics Technology littp:/..-’\\v'wx\'.ptonlinc.com/articles/200702fal .html, (accessed on May 12, 2007). Changchun, A., M. S, Synthesis, Structure and Properties of Polymer Nanocomposites, Ph.D dissertation, Chemical Engineering Department, Ohio State University (2004). ‘Martini-Vvedensky, J. S., Suh, N. P., and Waldman, F. A., Microcellular Closed Cell Foams and Their Method of Manufacture, US Patent 4,473,665 (1984). Park, C. B., Suh, N. P., and Baldwin, D. F ., Method for Providing Continuous Processing of Microcellular and Supermicrocellular Foamed Materials, US Patent 5,866,053 (1999). Rachtanapun, P., Microcellular Foam of Polymer Blends ofHDPE/PP and Their Composites with Wood Fiber, Ph.D. Dissertation, School of Packaging, Michigan State University (2003). 14. Naguib, N. E., Park, C. B., and Reichelt, N., J Appl. Polym. Sci, 91 (4), p. 2661- 2668(2004) Chapter 2 Literature Review The environmental issues such as global warming and municipal solid waste (MSW) disposed are causing more anxiety around the world. Municipal solid waste that is known as trash or garbage consists of product packaging, grass clippings, furniture, clothing, bottles, food scraps, newspapers, appliances, batteries, etc. Not included are materials that also may be disposed in landfills but are not generally considered MSW, such as construction and demolition debris, municipal wastewater treatment sludge, and non-hazardous industrial wastes [1, 2]. As shown in Figure 2.1, [2] the municipal solid waste in the US increased from 88.1 million tons in the 19605 to 151.6 million tons in the mid 19803, and continued to increase to 247.3 million tons in 2004, and 245.7 million tons in 2005. US. residents, businesses, and institutions produced more than 236 million tons of MSW in 2005, which is about 4.5 pounds of waste per person per day, which is only a slight change between 1990-2005. The current municipal solid waste management practices, as described by the US. Environmental Protection Agency (EPA), are the following [1]: - Source reduction, including repair and reuse of products or redesigning the products to use less material. - Disposal, including landfills, which is the most common way to manage MSW in the US. but has decreased over the last 20 years due to lack Of availability Of landfills, and combustion/incineration, which can reduce the waste volume [3] but is an expensive process. - Recycling, which can turn the waste into valuable resources; this process needs systems to collect and sort the materials and to manufacture and sell the recycled products. - Composting, which is controlled biological decomposition of organic matter, such as food and yard wastes, into carbon dioxide and water. MSW generation rates, 1960—2005 300 -- " 5 250 ‘* ‘i4 = 8 e . ‘13 200 -~ g 3‘. h ”K L. a ghs 150 SE 5 3.5 " .s g m _ q. .- 5: E ‘00 _ o —- -Total MSW eneration 2 3 £- 3 v / g ‘- C e o [.- —o—Per capita generation .. l :- 50‘r 0 i f I i i i 1 0 1960 1970 1980 1990 2000 2003 2004 2005 \kar Figure 2.1 Total waste generation and MSW generated per person in the US. from 1960- 2005l2l Plastics waste was about 12 wt % in MSW originated from goods in the US. in 2005, as shown in Figure 2.2. Some of this waste has a high degree of contamination, especially from food, and therefore it is difficult to recycle or reuse [4]. Biodegradable plastics could replace the conventional plastics and this waste could end up in composting facilities instead of landfills or incinerators. ASTM D 6400-04 defines “Biodegradable plastics are a degradable plastic in which the degradation results from the action of naturally occurring microorganisms such as bacteria, fungi, and algae and results finally in production of carbon dioxide and water” [5]. The biodegradable plastics are interesting from a sustainability viewpoint for composting containers such as fast food containers, which could reduce the volume of these one-time use containers in the landfills. Y‘s/”d , Other " 3% Glass 5% Rubber, Leather, 13:37 and Textiles o 7% Metals 8% Plastics 12% Yard Trimmings Food Scraps 13% 12% Figure 2.2 Total MSW generation in 2005 in the US. [2]. Degradable polymers “Degradable plastics are a plastic designed to undergo a significant change in its chemical structure under specific environmental conditions resulting in a loss of some properties that may vary as measured by standard test methods appropriate to the plastic 10 and the application in a period of time that determines its classification” from ASTM D 6400-04 [5, 6]. Degradable polymers are materials with inherent sensitivity to different degradation factors and can be degraded by physical, chemical, mechanical, or biological influences. [7]. Plastics are strong, lightweight, and inherently stable in the environment [8], but are easy to process and energy efficient. Plastics may achieve degradability using several types of degradation [7, 9] such as photodegradation, hydrolytic degradation, and biodegradation. Photodegradation is degradation resulting from exposure to light, especially UV light from the sun [9]. Hydrolytic degradation is degradation of plastics due to the action of water, including chemical reaction with water. Combining several degradation mechanisms can provide more efficient plastic degradation. Biodegradable polymers Biodegradable plastics commercially available in the market include [10]: - Starch based products such as thermoplastic starch, and starch derivatives; - Renewable resource based polyesters such as polylactic acid (PLA); - Naturally produced polyesters from bacteria including polyhydroxyalkanoates (PHAs), for example, polyhydroxy butyrate (PHB), and poly-E -hydroxy butyrate- co-valerate (PHBV); - Synthetic aliphatic aromatic polyesters, including Ecoflex® and Eastar Bio®; - Synthetic aliphatic polyesters, for example, poly e-caprolactone (PCL); and - Hydro-biodegradable polyesters (modified PET) such as Biomax®. ll The applications of biodegradable plastics include loosefill foam, food-service cups, trays, utensils, straws, films: such as mulch films for agriculture, composting bags, landfill covers, packaging, and flushable sanitary products [10]. Starch and derivatives Starch is produced in plants and consists of linear amylose (poly-oi 1, 4-D- glucopyranoside) and branched amylopectin (poly-0t l, 4-D-glucopyranoside and a-l, 6- D glucopyranoside) [11, 12]. The ratio of amylose and amylopectin varies depending on the sources of starch [11, 12]. Starch plastics are an inexpensive renewable material Starch from different plants such as corn, potato, or rice has differences in molecular size and shape, making it possible to choose an appropriate starch to suit each application [7]. The main problems of using starch are its hydrophilicity and brittleness. Glycerol and low molecular weight polyhydroxy compounds and poly(ethers) are common plasticizers used for processing starch [4]. The starch molecules have two important functional groups, 0H and COC. The OH group is susceptible to substitution reactions and COC is susceptible to chain breakage [8, 13]. For example, cross-linking or bridging of OH groups changes the structure of the network by reducing water retention and increasing resistance to thermo- mechanical shear [8]. Starch can be converted to a thermoplastic material, which replaces some synthetic polymers intended for short time durability or one time use [11]. Thermoplastic starch can be produced using gelatinization or destructurization. These processes involve disruption of the granular starch structure accompanied by liberation of amylose and amylopectin. Thermoplastic starch can be processed using conventional plastic 12 machinery, but because of its water sensitivity, it is difficult to process and use in some applications [1 1]. Novon, the first commercially available starch-based biodegradable plastic, was developed by Wamer-Lambert in 1990 [9]. The starch polymer contained about 30% amylose and 70% amylopectin, and used glycerol as a plasticizer. The company suspended the Operation of Novon in 1993 because of unsuccessful business. Then, Ecostar International acquired the technology and formed Novon International in 1995. Finally Churchill Technology acquired the technology, but the company filed for bankruptcy and Novon production stopped in 1996. Bastioli et a1. [7] patented technology based on destructurized starch, normally incorporating more than 60% starch by weight with a synthetic copolymer, eg. ethylene- acrylic acid, urea, or ammonia. The Italian company Novamont (part of the Montedison Co.) and the American company Novon (no longer in the market) produced starch-based materials under the Mater-Bi trademark [7, 9, 12, 14]. Mater-Bi has four classes, Z, Y, V, and A, with varying synthetic components [14] for specific applications. These polymers can be processed in conventional processing systems such as film blowing, blow molding, or extrusion. Mater-Bi has properties close to polyethylene (PE) and polystyrene (PS) [1 1]. This material is generally used in bag applications, especially bags for organic waste for composting [9], disposable items (plates, cutlery, cup lids), packaging wraps, personal care and hygiene [1 1]. EarthShell Corp. joined in a venture with DuPont to produce compostable materials such as cups, plates, bowls, and sandwich wrap using starch, limestone, a small 13 amount of fibers, and water to make laminated foams which were then coated with Biomax from DuPont. These materials physically disintegrate in water when they are crushed or broken [9, 15]. Polylactic acid (PLA) Polylactic acid is a biodegradable polymer from lactic acid, as shown in Figure 2.3. It is a highly versatile material and is made from 100% renewable resources such as corn, sugar beets, wheat, or other starch products. The renewable resources are milled to separate the starch from the raw material and then processed into unrefined dextrose. A fermentation process is used to turn dextrose to lactic acid. OH (pH H/(l:\C/CI;\H H H H 0 Figure 2.3 Molecular structure of lactic acid PLA was produced by Wallace Carothers, a scientist from DuPont, in 1932 using a low molecular weight product (lactic acid) to produce PLA by heating under a vacuum. Then in 1954, DuPont patented Carothers’ process [16] for the ring-opening polymerization process involving conversion of lactic acid tO a cyclic dimer [9, 17]. Because of high production costs, PLA was limited to use in medical applications. Then in the late 19805, a breakthrough in processing technology that decreased the costs of biologically produced materials such as lactic acid made PLA more interesting for non- medical applications [12]. In 1992, Cargill patented a PLA polymerization process using prepolymerization to low molecular weight polylactic acid, then conversion to lactide, and finally ring-Opening polymerization to produce PLA [l8]. Mitsui Toatsu [18] directly 14 converted lactic acid to high molecular weight PLA using a solvent-based process with azeotropic (where vapor and liquid have the same composition at the same point in distillation) removal of water by distillation. PLA is resistant to moisture and grease and also has flavor and odor barrier similar to polyethylene terephthalate (PET), which is used for soft drink and for food container products [19, 20]. Tensile strength and modulus of elasticity are also comparable with PET [20]. PLA can be produced as a rigid or a flexible polymer or as a copolymer with other materials [20]. PLA can be processed with conventional machinery such as thermoforming, injection molding, or sheet extrusion depending on the application [21]. PLA medical applications include extended release drug delivery systems. It is also used in packaging (film, therrnoforming containers, blister packs, short shelf-life bottles [9,12]. PLA can be also used in fibers and non-woven materials. C rystallinity, crystallization rate, transparency, and degradation rate of PLA are successfully controlled by manipulating the ratio of L and D isomers of lactic acid or lactide [12]. PLA has an ester group on the backbone, so it is sensitive to attack by non- specific esterases produced by microorganisms [7]. PLA is a nonvolatile, odor-free polymer and also is classified by the Food and Drug Administration (FDA) Of the United State as GRAS (generally recognized as safe) [19]. Polyhydroxyalkanoates (PHAs) Polyhydroxyalkanoates (PHAs) are renewable plastics which are synthesized biochemically by microbial fermentation. One of the most common 15 polyhydroxyalkanoates (PHAs) is poly(3-hydroxybutyrate) (PHB). Lemoigne, at the Pasteur Institute in Paris, first identified PHB in 1926. Commercial processes for PHA production were initially developed by W. R. Grace in the 19603 and later developed by Imperial Chemical Industries, Ltd, (1C1) in the United Kingdom in the 19703 and 19803. During the 19803, 1C1 developed a commercial process (large scale fermentation) to produce poly(3HB) and a related copolymer known as poly R-3-hydroxybutyrate-co-R—3- hydroxyvalerate, (poly(3HB-co-3HV)) that were commercialized under the trade name Of Biopol [22]. The main reason for developing these polymers was to replace petroleum- based plastics by renewable biodegradable plastics. In 1990, lCI transferred its agricultural and pharmaceutical businesses, including Biopol, to Zeneca [23]. In 1992, the DOE Plant Research Lab at Michigan State University genetically modified plants by taking two genes from PHB-making bacteria and inserted them into two cress plants and then crossed them to create a plant that could grow plastic [24]. In 1996 Monsanto bought all patents from Zeneca for making PHB [25]. In 2001, Metabolix, Inc., acquired Biopol from Monsanto and also developed it on a large scale. In 2004, Metabolix entered into a joint venture with Archer Daniels Midland to commercialize the fermentation processing technology based on renewable materials, for instance vegetable oil and corn sugar [9]. The same year, Metabolix started a joint project with the US. Army Natick Soldier Center to develop PHA for the Navy. Metabolix announced an alliance with British Petroleum in 2005 to develop the production of PHA in switchgrass [9]. PHB is an aliphatic polyester produced by a biological fermentation process in which polymers are produced inside the microorganisms [1 1]. PHB can be produced in 16 many bacteria. PHB has thermoplastic properties and can be processed in conventional machinery. It is not toxic and is totally biodegradable. PHB is a thermoplastic polymer with high molecular weight, and high crystallinity (about 60-80%). The melting temperature of PHB is about 175-180°C and its glass transition temperature is between 4-7°C [26]. The main drawbacks of using PHB are its brittleness and a narrow processability window (it tends to degrade near its melting temperature) [27]. PHB can be polymerized based on 3-hydroxybutyric acid (3HB), which is shown in Figure 2.4 [27]. / \ CH3 l/L/ 0 C n \10/ \an /_C)\ Figure 2.4 Structure of 3HB PHBs are natural thermoplastic polyesters, and hence the majority Of their applications are as replacements for petrochemical polymers currently in use for packaging and coating applications [28]. In Japan, PHB has found use in disposable razors, where the razor can be thrown into the toilet and rapidly degrades in sewage [24]. The use of PHB in medical applications has risen, especially in tissue engineering. PHB and P (3HB-co-4-HB) [4] can also be used as materials that slowly release drugs into the tissues in the body [24]. 17 Ecoflex® Ecoflex® is an aliphatic-aromatic copolyester based on terephthalic acid, adipic acid, 1,4—butanediol and modular unit [M] (components, e. g. monomers, with a branching or chain extension effect) as shown in Figure 2.5 [29]. O O 04134;}44 0—[CH2] rio—(ii) H“]—ICJ~ Figure 2.5 Structure of Ecoflex® Ecoflex® is a synthetic polymer that is fully biodegradable and compostable in compost facilities in about 180 days, in accordance with ASTM D 6400, EN 1342, the GreenPla standard from Japan, and the German standard DIN V 54900 [30, 31, 32]. Ecoflex® was designed to look and behave like low-density polyethylene (LDPE) and also can be processed on conventional LDPE lines. Ecoflex® can be blended with biomass-based materials such as celloluse, starch, polylactic acid (PLA), or biosynthetic polymers [30]. .Ecoflex® is already used in agricultural films that can be plowed into the soil after use, and in packaging as stretch wrap, drink cartons, and food packaging that can be composted [30]. Poly(t-: -caprolactone) (PCL) PCL is a thermoplastic biodegradable polyester that is synthesized by chemical conversion from crude oil and then ring-opening polymerization to produce a- caprolactone [7, 11, 12]. The properties Of PCL are good water and oil resistance, a low 18 melting temperature (50°C) and low viscosity [11, 12]. PCL has a modulus between LDPE and HDPE, and is a tough material at room temperature. PCL is used as a release material for controlled release of drugs [7]. PCL also can be blended with starch to increase water-resistance in trash bags [3 3]. PCL has been shown to degrade by hydrolysis in both soil burial and microbial testing chambers. The rate of hydrolysis or biodegradation of PCL depends on its molecular weight and degree of crystallinity. Biomax‘ID Biomax® is a material from DuPont, described as a hydro/biodegradable polyester and supplied in the form of pellets. Properties of Biomax® are shown in Table 2.1 and compared with PET. The material is based on polyethylene terephthalate (PET), and uses up to three different comonomers for incorporation into the polymer. These comonomers create weak spots in the polymer chains that make them susceptible to degradation through hydrolysis. Once the large polymer molecules have been broken down into smaller molecules by the action of moisture, naturally occurring microbes consume the small molecules, converting them into carbon dioxide and water [33]. 19 Table 2.1 Properties of Biomax® 4026 and PET. Properties Biomax® 4026* PET 1** PET 2*** Density (g/cnfl 1.33 1.29-1.40 1.3-1.33 '1‘g (°F and °C) 86 (30) 163-176 (73-80) 163-172 (73-78) L, 0F and °C) 392 (195) 473-509 (245-265) 469-482 (243-250) Tensile strength at break (psi (MPa)) 4600 (32) 7000-10500 (48.2-72.3) 7980 (55) Elongtion at break (%) - 30-3000 50-350 Izod Impact, Notched (lb/in or J/cm) 0.5 (0.27) - 2.62 (1.4) Note: Biomax® 4026 from DuPont [34]* PET l [35]** PET 2 from Matweb [36]*** Biomax® 4026 has a melting temperature (Tm) about 195°C (392°F), which is lower than PET 245-265°C. It is insoluble in water, has a specific gravity of 1.33-1.43, heat deflection temperature of 100°C (212°F) and a decomposition temperature of 340 °C (644°F) [34]. Biomax® can be formulated to give strength characteristics ranging from low-density polyethylene up to half the strength of polyester film [33]. Biomax® 4026 can be used as a film or coating in disposable food service packaging, such as bowls, plates, cups, sandwich wraps and clamshell sandwich containers, geotextiles, agricultural films, seed mats, plant pots, bags that cover ripening fruit, thermoformed packaging; blown bottles; and injection-molded objects. Biomax® can be manufactured in conventional PET equipment, so this makes the processing cost only slightly more expensive than PET [33]. However, the base resin cost is significantly higher (about $1 .80/1b [37]). 20 The Biodegradable Products Institute (BPI) awarded DuPont a “Compostable Logo” in June, 2003 for Biomax® 4026 hydro/biodegradable polyester resin [9, 10]. This symbol indicates that products made from Biomax® 4026 meet ASTM D6400-99 “Specification for Compostable Plastics” and are designed to biodegrade swiftly and completely, when composted in commercial or municipal facilities. Biomax® also has earned compostable approvals from DIN CERTCO in Europe and the Biodegradable Plastics Society in Japan. Biomax® can be recycled, incinerated or landfilled, but is intended mainly for disposal by composting and by degradation in soil [33]. Polymer foams Plastic foams can be defined as plastic materials with a cellular structure, in which there are voids or cavities, and therefore they contain at least two phases, a solid plastic matrix and a gaseous phase [3 8-42]. The solid polymer phase may be organic, inorganic or organometallic and also may consist of more than one solid. Other solids can be present in the form of fillers, which may be fibrous, glass, ceramic or metallic [40]. Plastic foams have some unique properties that differentiate them from solid polymers [43]. Polymer foams are widely used in a variety of applications because of their excellent properties: light weight, good strength-tO-weight ratio, good insulation properties, energy or material absorbing ability, and low cost [39, 41, 43]. On the other hand, foams usually have considerably reduced mechanical properties [39]. A variety of polymer foams such as polyurethane (PU), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), etc. have already been used for particular 21 applications [2, 5]. In 2001, in the US about 7.42 billion pounds of plastic foams were used, with a value of $16.2 billion [44]. The gas phase of polymer foams is distributed in voids, pores, or pockets called cells. If these cells are interconnected in such a manner that gas can pass from one to another, the foam is called open-cell. If the cells are discrete and the gas phase of each is independent of that of the other cells, the material is called closed-cell [41]. The closed- cell foams usually have lower permeability; therefore, these foams have better insulation properties than open-cell foams. 0n the other hand, open-cell foams have better absorptive capability [39]. Foams may be flexible, semi-rigid, or rigid, depending on their glass transition temperatures (below or above room temperature), chemical composition, degree of crystallinity, and degree of crosslinking [40]. Rigid foams are usually used in applications such as building insulation, cushioning, and food and drink containers. Flexible foams are generally used in fumiture, bedding, carpeting, and shock and sound attenuation [39]. Advantages of microcellular foams over conventional foams The main limiting factors in the production and application of conventional thermoplastic foams are the large size of the cells that characterize the materials and the lack of uniformity of those cells. Large, non-uniform cells decrease mechanical properties (e.g., strength, toughness, and fatigue resistance), introduce brittleness, and necessitate relatively thick part cross sections to ensure that the cells are contained within the material (thin cross sections are often flawed by breaks and holes) [45]. Conventional foam processes also can be inconsistent due to the inherent difficulty of controlling the 22 levels and release of the blowing agent (such as in the use Of chemical blowing agents, CBAs). Microcellular foams Microcellular foam is characterized by cell sizes in the range of 0.1 to 10 um, differentiating it from conventional cellular plastics with cells Of about 100 um and above [39, 46, 47], and has a cell population density in the range of 109-1015 cell/cm3. The unique features of microcellular foam are fine cell size, high cell population density, and use of inorganic blowing agents. In many cases, microcellular plastics exhibit high impact strength, high toughness, high stiffness-to-weight ratios, high fatigue lives, high thermal stability, low dielectric constants and low thermal conductivity [48]. But microcellular foams require very specific processing conditions, especially high pressure and high pressure drop rate [39]. Suh et a1. [46, 49] from the Massachusetts Institute of Technology (MIT) first developed microcellular foams in the early 19803 to reduce cost and weight without significantly sacrificing mechanical properties. Since the bubble size is smaller than the size of the flaw defects, the mechanical properties will not be reduced and at the same time, the material cost could be reduced [39, 43]. Microcellular foams can be produced by non-continuous processes such as batch processes and injection molding, continuous processes such as extrusion [39], or by semi- continuous processes. Martini and Suh developed the two-stage batch microcellular foaming process [43, 46, 49]. First, the polymer specimens are saturated with a non- reactive gas such as CO2 or N2 (CO2 is normally used because of its low cost and high 23 solubility in most plastics), in a high-pressure chamber with moderate pressure (500-800 psi) at room temperature [43]. At equilibrium, the polymer has absorbed gas to form a uniform polymer/ gas solution. Then the pressure is rapidly dropped to ambient pressure, causing supersaturation of the polymer/gas solution. Second, this supersaturated polymer/solution is heated up to a temperature high enough to soften the polymer. During this stage, a large number of bubbles are nucleated and grow into the microcellular structures of micrometer size. Then, the foaming samples are fixed by cold water to stabilize the cellular structure. As described, there are 3 basic steps in the microcellular foaming process: gas/polymer solution, cell nucleation, and cell growth [50-52]. The first step starts with an inert gas (usually CO2) absorbed into the polymer matrix at moderate to high pressure to form a uniform polymer/gas solution. Therefore, it will have a high gas concentration (typically 3-20% by weight). The solution depends on gas absorption and diffusion into the polymer matrix. The nature Of the polymer matrix, gas type, saturation pressure and temperature can affect the gas absorption and diffusion into the polymer [43]. The absorption and diffusion rate of N2 and Ar into polymers is much lower than CO2 [53]. CO2 is normally used as the blowing agent in almost all of the research on microcellular foams. The gas solubility and gas diffusion in polymers can be explained by Henry’s law and Fick’s law, as shown in equations 2.1 and 2.2, respectively [54, 55], C = 1&0. (2.1) —AE D=D ex ——D— 2.2 . p( RT) ( ) 24 where C is the equilibrium gas concentration, kS is the solubility coefficient, pS is the saturation gas pressure, D is the diffusion coefficient, D0 is the diffusion coefficient constant, AED is the activation energy for diffusion of gas in a polymer, R is the gas constant, and T is the absolute temperature. As can be seen from equation 2.1, the amount of gas in the polymer is directly proportional to the gas pressure. Increasing the pressure can increase the gas absorption and diffusion to facilitate the formation of the polymer/gas solution. As shown in equation 2.2, increasing the saturation temperature increases the gas diffusion rate into the polymer. However, this decreases the gas concentration at a specific pressure. In the batch foaming process, the polymer is usually saturated at low temperature (usually at room temperature) with high pressure to make sure to get high gas concentration in the polymer/ gas solution. In general, gas solubility in amorphous polymers is higher than in crystalline polymers (easier for gas to dissolve into the amorphous: less stiffness of polymer matrix). The basic microcellular process has been successfully applied to a number of amorphous polymers and some semi-crystalline polymers. Because of the slow diffusion of gas into polymers, it takes a long cycle time for the diffusion for gas into a polymer, usually from hours to several days, depending on the polymer’s properties [43]. The next step is the rapid nucleation of a large number of bubbles in the polymer/ gas solution due to a large thermodynamic instability. This can be accomplished by quickly changing the solubility of the gas by swiftly changing temperature or pressure. In this supersaturation state, gas molecules dispersed in the polymer/gas solution become energetic enough to overcome the surrounding confinement and expand. The nucleation 25 process is important, since it determines cell densities, cell morphologies, and the mechanical properties of the microcellular foams [43]. The nucleation in the polymer can be homogeneous, heterogeneous, or both. In homogeneous nucleating, nucleation sites are formed right through the mass Of the polymers at a molecular level. This homogeneous nucleation is driven by thermodynamic instabilities. [43, 45, 56] The homogeneous nucleation rate (Nhom) is given by equation 2.3 [57, 58], N = Cofo exp(—AG / kT) (2.3) hom horn where Co is the concentration of gas molecules, f0 is the frequency factor for gas molecules joining the nucleus, k is Boltzmann’s constant, and AG}mm is the activation energy for homogeneous nucleation. The activaton energy is given in equation 2.4 [5 8], 3 161w = —— 2.4 3m. -p.>2 ( ’ hom where y is the surface energy of the polymer; ps is gas saturation pressure and p0 is environmental pressure. A higher saturation pressure leads to a lower activation energy barrier as shown in equation 2.4, and accordingly, results in a higher nucleation rate (equation 2.3). f0 is the frequency factor representing the frequency of gas molecules joining the embryo nucleus and is related to the interfacial tension (ybp) and the mass of gas molecules m as shown in equation 2.5 [43] 2 1 f0 =(__y£)2 m (2.5) 26 Thus, the higher the gas saturation pressure is, the higher the cell density. Heterogeneous nucleation takes place at an interface between the polymer and another phase [56]. Heterogeneous nucleation sites typically involve additives that are added to lower the nucleating free energy, which usually consist of particulate solids such as talc or silica [43, 56]. The heterogeneous nucleation rate (N he.) is given by equation 2.6 [43] NM = flClexp(—AGhe,/kT) (2.6) where f1 has a similar physical meaning as f0 in homogeneous nucleating: the frequency factor for gas molecules joining the nucleus; and C1 is the concentration of heterogeneous nucleation sites, which is directly related to the particle concentration. Gibbs free energy for heterogeneous nucleating (Atht) is given by equation 2.7 [43] 16.7: , AG =—— ‘ 6 he! 3(Ap)2 prf( ) (2.7) where f(0) = (1/4)(2+c030)(1-c030)2 , 0 is the contact angle at the gas particle polymer interface. The final step is the growth of the stable nuclei, decreasing the overall density of the polymer matrix. The gas molecules from the polymer matrix diffuse into the nucleated cell and result in cell growth. Cell growth is generally controlled by the gas diffusion rate and the viscoelastic properties of the polymer/ gas solution. In general, a 27 higher foaming temperature and longer foaming time can enhance the cell growth process, because of reduced polymer matrix stiffness and increased gas diffusion rate. Ramesh [59] proposed a new experimental technique for studying the dynamics of bubble growth in thermoplastics using scanning electron microscopy. The effects of temperature, saturation pressure, molecular weight, and the nature of the physical blowing agent were investigated. The experimental results showed that the process variables control the growth of foams during processing. The existing Newtonian model for the growth of a single bubble in an infinite amount of polymer was modified to account for the non-Newtonian effects by modeling the polymer as a power law fluid. The experimental data was compared with the appropriate viscoelastic cell model which considers the growth of closely spaced spherical bubbles during the foaming process. The simulation results indicated that the predictions of the cell model were in qualitative agreement with the trends of the experimental data and the quantitative agreement was reasonable. The cell model also gave an equilibrium radius which agreed with the experimental data. Other viscous models do not predict the equilibrium radius of the bubble. Based on the fundamental process of producing microcellular foams, the microstructure of the foam and its properties could be customized by choosing foaming conditions to control the gas concentration, cell nucleation and cell growth processes for specific applications. The basic microcellular process has been productively applied to many amorphous polymers and several semi-crystalline polymers. 28 A huge disadvantage of the batch process is the long cycle time. This can be overcome in two ways. The first is use of supercritical C02 [60-63] to enhance the gas solubility and diffusion rate so the polymer is saturated at much higher pressure (usually 12-35 MPa), at a higher temperature (40-80°C). The high pressure can cause cell nucleation and cell growth at the saturation temperature, making it a one-stage process and resulting in much finer cell structure. Second, extrusion molding is used in the continuous process [64-67], which avoids the long cycle times required in the saturation process and can be developed using conventional industrial equipment. Microcellular foams produced by these methods have average cell sizes between 10-100 urn depending on the process conditions, and have been applied to many polymers, especially amorphous and a few crystalline thermoplastics, some thermosets, and liquid-crystalline polymers [68, 69]. Semi-continuous microcellular process The semi-continuous process is similar to the batch process: it involves gas saturation of polymers, and then passing them through a hot bath and a cold bath consecutively. The process, as shown in Figure 2.6, starts with a roll of interleaved polymer and a gas permeable material that are saturated with inert gas (usually CO2) in. a high-pressure vessel. After the interleaved materials are completely saturated, the roll is taken out of the vessel and the interleaved polymer separated from the gas permeable material. The saturated polymer sheet is drawn into a hot bath and then into a cold bath to make the desired foam [53, 70, 71]. 29 Gas Saturated Polymer Roll Foam Roll Hot Cold Paper Bath Bath Towel Figure 2.6 Semi—continuous microcellular process [69]. Continuous microcellular process Due to some disadvantages such as the long time for the absorption process, and poor cost effectiveness of the batch process, the continuous process was developed with the same concept of thermodynamic instability [49, 72, 73]. In the continuous microcellular process, the need for time to dissolve gas into the polymer is reduced. Continuous processing was developed at MIT and was scaled up to a continuous commercial process in 1994 by Trexel, Inc. under the trade name MuCell [74, 75]. The continuous process is also based on the concept of thermodynamic instability [76]. Supercritical fluids (usually CO2) are injected into a polymer melt in an extruder barrel under specified pressure profiles. Then a homogeneous polymer solution is ejected through the die, creating a rapid pressure drop [9, 77]. Microcellular foam is created by cell nucleation and growth under the rapid pressure drop in the in-line die. 30 Blowing agents Blowing agents are the particular agents which cause plastics to foam. The most general classification of foaming agents is based on the mechanism by which gas is generated. Therefore, blowing agents can be classified in two types. Compounds that liberate gases as a result of physical processes (evaporation, desorption) at elevated temperatures or reduced pressure are called physical blowing agents (PBAs). PBAs do not undergo chemical transformation themselves, and most of them are liquids [3 8]. Chemical blowing agents (CBAs) are individual or mixtures of compounds which release gas as a result of chemical reactions or thermal decomposition, resulting from chemical reaction, or interaction with other components of the formulation. Most CBAs are solid and experience chemical transformation during the process of foaming [78]. CFCs (chlorofluorocarbon liquids), n-pentane, and n-butane in conventional foaming processes [79] used to be the most commonly used blowing agents due to their high solubility in the polymer resin [80], so a foam product with high void fraction can be produced at a relatively low pressure [79]. However, CFCs are linked to depleting of ozone in the upper atmosphere, and since 1987, this use has been stopped by the Montreal Protocol on Substances that Deplete the Ozone Layer [81]. The use of hydrocarbon blowing agents such as n-butane is also not preferable because of their high flammability [82]. Due to the urgent global ban on CFCs, inert gases have been considered as substitute blowing agents, and the most common gases used are CO2, N2 and Ar [83]. These gases are dissolved under pressure in the resin, and when the pressure is reduced, the gases become less soluble in the polymer and form cells. 31 Chen et a1 [84] studied the effects of gas types on the microcellular foaming process using gases such as CO2, N2 and Ar, as blowing agents in HDPE and PVC. A foaming process simulator and an extruder with a capillary die were used for the test. They found that gas absorption, diffusion rate, and viscosity reduction were lower in N2, and Ar compared to CO2, and the average cell size was bigger with CO2 than with N2 and Ar. On the other hand, the cell nucleation density was similar for all three gases at high saturation pressure, but it was significantly lower with N2 or Ar when the saturation pressure was low. Carbon dioxide and its interaction with polymers CO2 is a chemical compound consisting of one carbon and two oxygen atoms. It has a density of 1.98 kg/m3 at 25°C, which is about 1.65 times that of air [85]. The use of CO2 as a solvent in foaming processes has undergone rapid development [86]. The critical point of CO2 is relatively low (3 l.l°C and 7.38 MPa) and easily attained. CO2 has liquid-like solubility and diffusivity and gas-like viscosity at supercritical conditions. Compared to organic solvents, CO2 has vast attractive properties such as high diffusivity, so it is possible to dissolve sufficient CO2 in a polymer melt quickly. It can reduce the viscosity and surface tension of polymer melts, which assists many polymer processing Operations [87]. It is low cost, non-toxic, non-flammable and environmentally friendly [79,88], making it an excellent agent for use in foaming. In order to understand the polymer foaming process using CO2, it is essential to understand the interaction between CO2 and polymers, and how these interactions change the physical properties of polymers that are important in the foaming process, for example, viscosity, interfacial tension glass transition temperature, or melting temperature. 32 Solubility of C02 in polymers The solubility of CO2 in polymers is significant since it determines the minimum amount of gas for the foaming process. Additionally, the amount of C02 dissolved in the matrix affects the viscosity and interfacial tension, which have a huge influence on the foaming process. CO2 is usually considered as a Lewis acid (electron acceptor) [88]. Polymers with electron donor groups such as carbonyl groups, ether groups, or fluoro groups can interact with CO2 using Lewis acid-base interaction in which a Lewis acid is an electron pair acceptor and a Lewis base is an electron pair donor. The simplest reaction is for a Lewis acid (A) to interact with a Lewis base (B) to give a Lewis acid/base complex (A-B): A+B -> A-B [89]. The solubility of CO2 increases with an increasing content of polar groups in the polymer. Non-polar polymers such as polyolefins lack this interaction and the solubility of C02 in such polymers is very low [88, 90]. Fourier transform infrared spectroscopy (F TIR) was used to study the interaction between CO2 and various polymers [88, 90]. Especially, the bending mode of CO2 was used to probe polymer-CO2 interaction. Polymers with carbonyl groups display complimentary interaction with CO2 demonstrated by the splitting, shift and broadening of the band associated with the bending mode Of CO2. Polymers lacking electron donor properties do not show the splitting. Wissinger et a1. [91] studied the solubility of C02 in PS and PMMA over a temperature range from 33 to 65°C and pressure up to 100 atm. A linear relationship between C02 solubility and saturation pressure was observed. Handa et al. [92] examined CO2 solubility in PMMA in the temperature range from 0 to 167°C and pressure up to 33 61.2 atm and found that the solubility was convex to pressure at low temperature, but approached a linear relationship with pressure at high pressure. Hougliu et al. [93] studied poly(ether sulfone) (PESF) and poly(phenylsulfone) (PPSF), investigating the effect of processing conditions on the foam densities and microstructures of foams. These polymers were saturated in a vessel with CO2 at a pressure of 810-820 psi at room temperature for 70 hours. The C02 uptake was determined from CO2 desorption curves, and was found to be 7.8 weight % for PESF and 8 weight % for PPSF. They also studied the effect of processing parameters such as saturation time, pressure, foaming time and foaming temperature on the relative densities and microcellular structure of polysulfone (PSF), poly(ether sulfone) (PESF), poly(phenylsulfone) (PPSF), polyetherimide (PEI), and polyether ketone ketone (PEKK) [94]. The CO2 sorption in polymers was studied as a function of saturation time at 830 psi CO2 at room temperature. The CO2 uptake in these polymer matrices increased rapidly in the early stages and reached equilibrium at 60 hours for PSF at 10 wt%, PPSF at 9 wt% and PESF at 8 wt%, but for PEI and PEKK it did not reach equilibrium even afier 120 hours, with CO2 content of 9 and 2.4 wt %, respectively. They also studied PSF relative density as a function of CO2 content at 175°C foaming temperature, and 30 sec for foaming time. It was shown that with an increase in CO2 content in the polymer matrix, the relative density decreased. The more CO2 diffuses into the polymer, the more bubbles can be nucleated; therefore, the lower the foam density. SEM results showed that when C 02 content was less than 4% by weight (a saturation time of about 16 hours) the middle part of polymer could not be foamed. This meant that CO2 did not diffuse into the middle of the 1.5 mm thick PSF sample. When 34 CO2 content was higher than 6.5% by weight (about 24 hours sorption time), the relative density remained almost constant. This implied that the requirement for the sorption time was at least 24 hours or 6.5% CO2 content to get a uniform gas/polymer solution for the PSF foaming process. Li et al. [95] used a magnetic suspension balance to measure the sorption uptake of supercritical fluids of CO2 and N2 in melted PLA. The Sanchez—Lacombe (SL) and Simha-Somcynsky and equations-of—state (SS-EOS) were used to predict volume swelling determination of the solubility. This showed that there were, significant differences in the solubility of CO2 and N2 in the melted PLA. The CO2 and N2 solubility in melted PLA were 20% and 2%, respectively. Manrinen et al. [96] studied TS and sorption and diffusion of subcritical CO2 in poly(methyl methacrylate) (PMMA) nanocomposites containing organically modified smectite clay (cloisite 20 A). CO2 solubility was studied from 0 to 65°C with pressures up to 5.5 MPa (800 psi) using a gravimetric technique and compression molded films. They found that the organo-clay had no effect on the solubility of CO2 in PMMA. Therefore the solubility of CO2 in nanocomposites can be determined from the solubility of CO2 in the polymer matrix. Effect of processing conditions on microcellular foams Some parameters such as solubility of gas in the polymer matrix, the rate of gas diffusion in and out of the cells, the interfacial surface energy, the viscoelastic properties of the polymer/ gas solution, and the amount of gas loss during the foaming process affect the foaming ability [48, 49, 97-99]. These parameters are also related to the foaming conditions (foaming time, foaming temperature, saturation time, saturation pressure) [97, 35 98, 100], the amount of crystallinity [101], and phase heterogeneity (additives, plasticizers). Doroudiani et al. [102] investigated the processing conditions for expanded polystyrene (EPS) using carbon dioxide near the critical point as a blowing agent. They showed that the foaming time was the most important factor in determining cell size and cell density. EPS foams can be produced having the same densities with different cell sizes by controlling the foaming conditions. The effect of foaming conditions, especially saturation time, saturation pressure, foaming time and foaming temperature, on nucleation and cell grth of both amorphous and semi-crystalline PET with and without a polyolefin nucleation agent in batch processing was studied by Baldwin et al. [97, 99]. They found that the gas saturation time used should be at least the minimum time to reach a uniform solution. The cell density increased with decreasing cell size, and with increasing saturation pressure. The relationship between foaming temperature and cell size and cell density for microcellular foamed poly(ethylene terephthalate) and for PET containing a polyolefin nucleating agent was investigated by Shimbo et al. [103]. They found that increasing foaming temperature increased the average cell size, but cell density was almost constant. In addition, the relative density increased with decreasing average cell size. The effect of saturation temperature on the polycarbonate-CO2 system was studied by Weller and Kumar [104]. They reported that foam density was not influenced by foaming temperature. However, the saturation temperature significantly affected cell nucleation density and cell diameter. The cell nucleation density decreased and average cell diameter increased when saturation temperature increased above approximately 80 36 °C because of a change in free volume. Kumar and Weller [105] reported that cell density of foamed polycarbonate (PC) increased with saturation pressure but was independent of foaming temperature over a wide temperature range (60-160°C). Parks and Beckman [106] studied the effects of phase separation conditions on polyurethane microcellular foams. They found that the higher the CO2 pressure, the more CO2 was provided for foaming, generating lower interfacial tension and viscosity in the polymer matrix and therefore producing higher cell densities. Effect of crystallinity on microcellular foams In batch processing, semi-crystalline polymers are generally more difficult to process into foams because cellular structures are not uniform due to the heterogeneity of the material [100, 107]. Baldwin et al. [100] studied the effects of crystallization in semi-crystalline and amorphous PET in microcellular foaming using a batch process. Crystallization of the polymer matrix resulted in lower solubility due to the lower diffusivity and higher polymer matrix stiffness. They also reported that increasing the saturation pressure increased gas concentration in the polymers, induced crystallinity from the plasticizing effect of gas at high concentrations, and decreased the glass transition temperature due to the plasticizing effect. They also determined that semicrystalline PET polymers were more difficult to foam than amorphous PET since they needed higher temperature to foam. However, the semicrystalline PET foams had higher cell density and smaller average cell size than amorphous PET [100]. They concluded that the crystallinity of the PET polymer matrix played a main role in the process of foaming. In the semicrystalline PET, foaming tended to be governed by viscoelasticity of the CO2/PET solution, but in 37 the amorphous PET, foaming was most affected by the diffusion of the gas. In semicrystalline PET foam, cells nucleated at the interface between the amorphous and crystalline regions due to lower activation energy. Semicrystalline polymer foams had a smaller average cell size because of the increase in stiffness of the polymer matrix. However, increasing the foaming time and temperature increased the cell size for semicrystalline PET [100]. Doroudiani et al. [101] studied the effects of crystallinity and morphology on the microcellular foam structure of semicrystalline polymers such as HDPE, PP, PET, and polybutylene (PB) by using varied cooling rates to control the crystallinity in the polymers. They found that increasing the crystallinity of the polymer decreased the solubility and diffusivity of the gas. With a slow cooling rate, the polymers had high crystallinity and non-uniform cell structure, with a lower amount of gas diffused into the polymer matrix. On the other hand, with a fast cooling rate, the polymers had low crystallinity and high gas solubility and a uniform and finer cell structure. They also concluded that the morphology of polymers strongly affected the solubility and diffusivity of gas. Colton [108] studied nucleation phenomena in PP with and without nucleating agents, as well as in copolymers of PE/PP. He found that there was low gas solubility in the PP and PE, as gas could not dissolve in the crystalline regions. In order to overcome this problem, he recommended using a foaming temperature above the melting temperature and adding appropriate nucleating agents in the polymer, or using a PP/PE copolymer to increase the amorphous regions. 38 Properties of polymer microcellular foams In PC [109] and PVC [110-113] microcellular foams, both tensile strength and yield strength are proportional to the foam density. Zhang et al. [114] studied the effects of molecular weight and foam density of HDPE microcellular foam on mechanical properties during tension, and at the break point. They found that increasing the molecular weight of polyethylene changes the tensile behavior from brittle to ductile. The toughness of foams was found to increase with normalized density and molecular weight of HDPE. Doroudiani et al. [115] investigated the effects of processing conditions and structure on the tensile properties of EPS. Foaming time and foaming temperature were the most important processing parameters influencing the tensile modulus and strength. The tensile modulus and strength increased with increasing foam density, but they decreased a little when the cell size increased. Toughness was determined for various polymers. Notched Izod impact strength of PC microcellular foams was reported to be higher than solid PC when the relative density (foam density/solid polymer density) was higher than a certain value [116, 117]. The impact strength decreased with a decrease of density (linear function) or increase of cell size. PVC microcellullar foams using the solid-state process (batch process) with C02 as a blowing agent with relative densities of 0.6 and higher were processed and impact tested using a falling-weight impact tester by Juntunen et al. [118]. They found that the impact strength of microcellular PVC foams decreased linearly with the relative density, 39 and the gas saturation pressure did not significantly affect the impact strength of the foams. Doroudiani and Kortschot [119] investigated the effects Of processing conditions and structure on impact strength of foamed EPS. The foaming temperature was the most important to determine the specific impact strength, and the impact strength increased with an increasing relative density. At the least relative density of 0.6, the impact strength was double of the neat EPS. The tensile and impact strength of PS, PC and SAN microcellular foams were studied [120-122]. Microcellular PS foams only showed trivial improvement in tensile strength and impact toughness, while PC microcellular foam with a cell size of 40 um and 28% void fraction microcellular showed substantial increase in Izod impact strength. Barlow et al. [117] studied the effects of density, cell size, and residual gas content on the impact strength Of solid-state microcellular polycarbonate foam. They indicated that the impact strength of the foamed samples decreased linearly with density. Waldman [123] studied the impact strength of EPS microcellular foam, and found a greater impact strength for microcellular foam compared with neat EPS. Collias and Baird [120] investigated the impact behavior of microcellular EPS, styrene acrylonitrite copolymer, and polycarbonate. They found no improvement in the impact strength of expanded samples compared to neat polymers. Doroudiani et al [102] investigated the effects of processing conditions and structure on impact strength of EPS. The foaming temperature was the most important to determine the specific impact strength. 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E., Ma, M., Montecillo, R., and Kwapisz, R. R., Cell. Polym., 17, p 350 (1998). 112.Matuana, L. M., Park, C. B., and Balatinecz, J. J., SPEC-ANTEC, 56, 1968 (1998) 1 l3. Matuana, L. M., Park, C. B., and Balatinecz, J. J., Cell. Poly., 17, p 1 (1998). 114. Zhang, Y., Rodrigue, D., and Ait-Kadi, A., J. Appl. Polym. Sci., 90, p 2130-2138 (2003) 115. Doroudiani, S. and Kortschot, M. T., J. Appl. Polym. Sci., 90, p 1427-1434 (2003) 1 l6. Barlow, C., Kumar, V., Flinn, B., Bordia, R.K., and Weller, J., 82, Porous, Cellular and Microcellular Materials, ASME (2001). l 17. Barlow, C., Kumar, V., Flinn, B., Bordia, R. K., and Weller, J., J. Eng. Mat. Tech, 123, p 229-233, (2001). 1 18. Juntunen, R. P., Kumar, V., and Weller, J. E., J. Vinyl. Additive Tech, 6, no 2, p 93-99, June (2002). 1 l9.Doroudiani, S. and Kortschot, M. T., J. Appl. Polym. Sci., 90, p 1421-1426 (2003) 120. Collias, D. 1., Baird, D. G., and Borggreve, R. J. M., Polymer, 35 (18), p 3978- 3983(1994) l2l.COllias, D. 1. and Baird, D. G., Polym. Eng. Sci., 35 (14), p 1167-1177(1995). 122.Collias, D. 1., and Baird, D. G., Polym. Eng. Sci., 35 (14), p 1178-1183 (1995). 49 123. Waldman, F., MS. Thesis, Massachusettes Institute of Technology, Cambridge, MA (1982). 50 Chapter 3 Materials, Methods, Sample Preparation, Sorption Test, and Characterization of Samples and Foamed Samples The condition or state of the polymer phases, including the orientation, crystallinity, previous thermal history, and chemical composition, determine the properties of the phase [1]. Polymer state and cell geometry are intimately related because they are determined by forces exerted during the expansion and stabilization of foams. Preparing the samples for foaming is an important process due to the effects on the foamed properties: density, average cell size, or cell population density. In this study, two methods for preparing Biomax® specimens were compared. First, samples were prepared using a conical counter-rotating twin-screw extruder through the die (1.5 inch width and 3/8 inch thick) to make 3/8 inch thick samples which were cut into 2 inches long, compression molded to make 1.5 mm thickness sheet, and then cut into 1 in x 0.5 in ® (length x width) samples. Second, samples of Biomax were prepared using a micro- compounding machine (mini twin-screw extruder with injection molding), and then cut into samples of 0.475 in width x 1.15 in length (half of the bar) with 2.12 mm thickness. These two types of samples were foamed after saturation in a C02 high-pressure chamber and the results compared. The structure of foams depends on the quantity of the blowing agent (usually CO2) dissolved into the polymer, so the solubility is a very important property [2]. The CO2 uptake in the sorption experiments was measured by periodically removing samples 51 from the vessel chamber and immediately weighing them [3]. This method was used to study the saturation time for CO2 uptake in Biomax® specimens. Materials and methods Materials Biomax® 4026 was provided from DuPont in pellet form. Glycerol was from IT. Baker (Phillipsburg, NJ, USA), and 99.5% CO2 gas from Linde Gas (Lansing, MI, USA). Methods Characteristics of Biomax® pellets A therrnogravimetric analyzer (TGA) was used to measure the decomposition temperature of polymers. Differential Scanning Calorimetry (DSC) was used to measure the glass transition temperature (Tg), the melting temperature (Tm) and heat of fusion of polymers. TGA (Thermogravimetric analyzer) A TGA 2950 from TA Instruments (Delaware, USA), as shown in Figure 3.1, was used to measure the weight loss and decomposition temperature of Biomax® 4026 from 35-500°C with a heating ramp of 20 °C/min under a nitrogen atmosphere. As can be seen in Figure 3.2, Biomax® resin starts to decompose at 350°C. At a pellet temperature of about 359°C, the weight loss is about 1%. 52 Weig ht (°/o) Figure 3.1. TGA 2950 from TA Instruments. 380.54°C 0 04481%r"C 160 260 300 Temperature (°C) Figure 3.2. TGA results of Biomax® pellet 53 - ‘r 400 Deriv. Weight (%/°C) DSC (Differential Scanning Calorimetry) A DSC Q 100 from TA Instruments (Delaware, USA), shown in Figure 3.3, was used to measure the glass transition temperature (T8), the melting temperature (Tm), and the heat of fusion. The heat cool heat method was used to reduce the heat history with the cycles from —30 to 250°C (heated from -30 to 250°C then cooled down to -30°C, and heated up to 250°C) with a heating ramp of 10 °C/min under a nitrogen atmosphere. The Tg, Tm, and heat of fusion of Biomax® pellets are about 36°C, 195°C, and 21 J/g, respectively, as shown in Table 3.1 and Figure 3.4. Figure 3.3 DSC Q 100 from TA Inst z ruments. Table 3.1. The glass transition temperature, melting temperature, and heat of fusion of Q1) Biomax pellets. Samples Tg Tm AHr (°C) (°C) (J/g) 1 35.87 195.69 19.5 2 36.09 193.92 22.5 Average 35.98 194.805 21.0 ST DEV 0.16 1.25 2.12 54 30 20 ’5) E i E, 10 - LL ‘6 a) I i 0 5 31 08”C 160 49°C 35.59~c11) 2263.119 38 (we ' r N 192250 '10 r r r ' r r r c r r r ' r r ' r ' r r _ .50 o 50 100 150 200 230 Temperature (°C) Figure 3.4. DSC result of Tg, Tm, and AHf. Preparing Biomax® specimens. Twin-screw extruder An extruder is used to convert the solid plastic (pellets) into melted plastic using heat and pressure [4]. Then the melted plastic leaves the extruder through a rectangular shaped die, and is cooled by air. In this experiment, Biomax® pellets could not be directly converted into sheet using compression molding alone, since the outside surface of the sheet was burned before the pellets uniformly melted to produce the sheet. 55 Twin-screw extruder conditions In this experiment, a conical counter rotating twin—screw extruder with L/D ratio of 13:1 (C. W. Brabender Instruments, Inc., South Hackensack, NJ, USA) with a die was used to make 3/8 inch thick by 1.5 inch width specimens. The temperature conditions were 185°C for zones 1 and 2, and 190°C for zone 3 and the die, with the screw speed at 30 rpm. The samples were cut into 2 inch lengths. Compression molding Compression molding (Carver Laboratory Press, Model M, Menomenee Falls, WI, USA), as shown in Figure 3.5, is one of the oldest techniques for polymer processing. In general, in this process, pellets, sheets or films of plastics are placed in the frame [5]. Heat and pressure is used to melt the plastics. The advantages are modest amounts of deformation, and there are no regions of very high stress, such as the gate of an injection mold. Figure 3.5. Compression molding 56 Compression molding conditions After extrusion, two pieces of the 3/ 8 inch sheet were used in compression molding. Molding conditions were 200°C in both upper and lower parts, for 45 see without pressure and then under a pressure of 15,000 lb force for 1 min. The samples are shown in Figure 3.6. The samples were light grayish, Opaque, dense, stiff, brittle, and the edges were a little rough from cutting. There are referred to as Sample I. Figure 3.6. Sample from compression molding, and cut samples Mini twin-screw extruder and injection molding The mini twin-screw extruder and injection molding system, L/D: 18:1 and length 150 mm (Micro 15, DSM Microcompounding Machine, DSM Research, Netherlands), as shown in Figure 3.7, is a small twin screw extruder (only one inch diameter). Polymer is melted in this extruder and then the cylinder (set at the same temperature as the bottom part of the extruder) is used to receive the melted polymer and then pressure is used to move the melted plastic from the cylinder to the mold. 57 Mini twin-screw extruder conditions: Temperatures of all extruder sections (top, middle, and bottom) were 210°C with the screw speed at 100 rpm. The injection mold was at a temperature of 23°C. The samples were molded into a rectangular shape as shown in Figure 3.7 d, left. The samples then cut into 0.475 inch width x 1.15 inch length (half of the bar) with 2.12 mm thickness. The samples were translucent, light brown, and a little flexible, with a good surface finish. These are referred to as Sample 11. 58 a) Twin-screw extruder c) Molds (1) Samples Figure 3.7. Mini-extruder with injection molding system with its molds; a) twin-screw extruder, b) injection molding system, c) molds, and d) samples. 59 Foaming process: batch process Samples from the two methods were placed in a C02 high-pressure vessel (850 psi or 5.86 MPa) at room temperature. CO2 is normally used as the foaming agent due to its low cost and high solubility in most plastics. The samples absorbed gas over time until they were saturated, about 2 days. The saturated samples were removed from the high- pressure vessel and heated in a hot glycerol bath at the specified foaming temperatures and foaming time. A large number of bubbles were nucleated and grown by inducinga large thermodynamic instability related to quickly changing the solubility of the gas by decreasing pressure and increasing temperature. After foaming, the foamed samples were quenched in cold water to stabilize the microcellular structures. Comparison of processing conditions Foaming temperatures of 100, 105, 110 and 120°C and time of 5, 10, 15 and 20 see were used for sample I. For sample 11, the foaming temperatures of 100 °C for 5, 10, 20, and 30 sec, and 120°C for 5 and 15 sec were used. Twin-screw extruder with compression molding (Sample I) The Biomax® samples from the twin—screw extrusion and compression molding were slightly grayish, opaque and stiff, and appeared to have higher percent crystallinity than Sample 11 (brownish, translucent, more flexible). At the foaming temperature of 100°C, as shown in Figure 3.8 a, samples did not foam at 5 or 10 sec foaming time. They still had the same grayish color and the size of the foamed samples was still the same. At 15 and 20 sec, the samples were foamed and were white and stiff, with a smooth surface. 60 At the foaming temperature of 105°C, as shown in Figure 3.8 b, the middle part of the sample did not foam at 5 sec, as can be seen by the difference in color between the middle (slightly grayish) and the outside (foamed sample: white). At 10, 15, and 20 sec, the samples foamed well, resulting in a white smooth surface, and were stiff. At the foaming temperatures of 110 °C, as shown in Figure 3.8 c, and 120 °C as shown in Figure 3.8 d at 5, 10, 15, and 20 sec foaming time, the foamed samples had bubbles at the surface indicating cell collapse. The Biomax® samples were observed to lack uniformity, as they sometimes had some small holes or bubbles and slightly varied in thicknesses. 61 c) 110°C (1) 120°C Figure 3.8 Foamed samples of sample I at a) 100, b) 105, c) 110, and d) 120 °C foaming temperatures at the foaming times of unfoamed, 5 sec, 10 sec, 15 sec and 20 sec, from left to right respectively. Mini twin-screw extruder with injection molding (Sample 11) At the foaming temperature of 100°C at 5 sec foaming time, the samples were foamed but were not foamed in the middle. At 10 see, the samples were foamed but remained slightly brownish. At 20 sec, the samples were foamed well, with a uniform and white smooth glossy surface. At 30 see, the samples had bubbles on the surface indicating too long a foaming time, as shown in Figure 3.9 a. 62 At the foaming temperature of 120°C and 5 sec foaming time, the foamed sample had a smooth white surface. At 15 see, the samples foamed well, with a white smooth glossy surface, as shown in Figure 3. 9 b. a) 100°C b) 120°C Figure 3.9. Foamed samples of sample H at a) 100°C at the foaming time of unfoamed, 5, 10, 20, and 30 sec, left to right respectively, and b) 120 °C at unfoamed, 5, and 15 sec foaming time, from left to right. Comparison of foams from the 2 methods The best processing condition of each method, was used to compare the methods. For sample I, the best condition was foaming temperature of 100°C and foaming time of 20 sec. As shown in Figure 3.10 a (left), the unfoamed sample was slightly grayish, dense, and stiff. The foamed samples, as shown in Figure 3.10 a right, were white with a smooth surface, but still relatively dense. For sample 11, the best foaming temperature was 120°C and foaming time was 15 sec. The unfoamed sample 11, as shown in Figure 3.10 b left, was slightly brown, somewhat flexible, with a smooth surface. Foamed samples (Figure 3.10 b right) had a 63 white, smooth glossy surface, were flexible, had high volume expansion and uniform bubbles. Figure 3.10 c shows a comparison between foamed samples of I and 11. Sample 11 (left) had the better foams: white, smooth glossy surface, flexible, more volume expansion and uniform bubbles. The preparation methods of the samples affected the foaming ability of Biomax® samples. Preparation of Sample 11 was simpler, involving only one process and one heat history, Sample I required two steps and two heat histories. Especially, compression molding was difficult to achieve within the needed narrow temperature range (high enough temperature for heat to diffuse into the centerline, but not too high to cause burning of the outside surface layers). Sample I appeared to have more crystallinity than sample 11, as evidenced by the opaque surface and increased stiffness. The cooling in compression molding is slower, using water to cool the hot plates, compared to injection molding where the mold is maintained at the cooling temperature. The increased crystallinity of Sample I likely retards or reduces CO2 absorption, which may contribute to reduced foaming. Sample I does not have any significant orientation in the samples (compression molding) whereas sample 11 (injection molding) is expected to have orientation in the flow direction. Because the mini-extruder with injection molding (sample 11) produced better foams, it was used to prepare the samples for the remaining experiments. 64 Sample I Sample 11 a) Unfoamed (left) and foamed (right) b) Unfoamed (left) and foamed (right) c) Unfoamed and foamed (left and right) of Sample 11, and Sample I on the left and right, respectively Figure 3.10 a) Unfoamed and foamed sample I, b) Unfoamed and foamed sample 11, c) Unfoamed and foamed samples of sample 11 and I from left to right. For sample I, the foaming temperature was 100°C and foaming time was 20 sec. For sample 11, the foaming temperature was 120°C and foaming time was 15 sec. 65 The saturation time of CO2 uptake of Biomax® specimens The structure of foams depends on the quantity of the blowing agent (usually CO2) dissolved into the polymer, so the solubility is a very important property [2]. Materials Biomax® 4026 samples were prepared using the mini-extruder with injection molding into a rectangular shape and then cutting the samples into 0.475 in width x 1.15 in length (half of the bar) with 2.12 mm thickness. Temperatures of all extruder sections (top, middle, and bottom) were 210°C with the screw speed at 100 rpm. The injection mold was at a temperature of 23°C. Methods Biomax® samples from sample 11 were weighed and then placed in the CO2 vessel chamber for the specified time of l to 6 days. After the specified times, the samples were removed and were reweighed. Experiments were performed in duplicate. After that, the saturated polymers were foamed at 120°C foaming temperature with 10 sec foaming time. Selection of sorption conditions From Table 3.2 and Figure 3.1 1, it can be seen that sorption of CO2 in Biomax® polymers quickly increased in the early stage, until reaching saturation between 3 days (7.28%) and 4 days (7.3%). Two days saturation time was used for the remaining experiments because there was less than 0.2% difference in CO2 between 2 and 3 days, which was not significant. The foamed sample with saturation time of 1 day, as shown in Figure 3.12, did not foam well. It still had some middle parts that did not foam, and was stiff, probably 66 because of lower CO2 absorbed (only 5.33% weight gain). The foams with saturation times of 2-6 days foamed well, had white smooth surfaces, and uniform bubbles, as shown in Figure 3.12. A saturation time of two days was selected for the experiment, resulting in about 7.11% weight gain. Table 3.2 Results of weight gain at the different saturation times in the CO2 high-pressure chamber Days % Weight gain 0 5.333014 7.11:l:0.08 7.28:l:0.22 7.30:l:0.22 7.05i0.25 7.13:l:0.44 O\UIAMNH¢ 67 Weight gain (%) 7.11 7-28 7.05 7.13 ° ' 5.33 4 . 2 u o e . . . . . . . o 1 2 3 4 s 6 7 Time (Day) Figure 3.11 co2 uptake in the Biomax® matrix Figure 3.12 Samples of unfoamed Biomax® and foams at 1, 2, 3, 4, 5, and 6 days of saturation time from left to right 68 After preparation Of Biomax® specimens and production Of foams in the batch process, the foamed samples were characterized and tested. Characterization of foams Dimensions A Vernier caliper was used to measure the sample dimensions. At least five samples were measured and the data averaged. Density The density of the samples was measured by a water displacement technique following ASTM D-792, using equation 3.1 [10, 11] M Density = 0.9975 Ma (3.1) W where Ma is the weight of unfoamed or foamed samples measured in air, and MW is the weight of unfoamed or foamed samples measured in distilled water. At least five samples were measured and the data averaged. Void fraction The calculation Of void fraction (VF) was determined using equation 3.2 [10, 1 l] VF = - — (3.2) where p is the density Of the unfoamed sample, and pr is the density of the foamed sample. 69 Morphology analysis The foam samples were frozen in liquid nitrogen for 10 min and then fractured and inserted into the sample holders (stubs). The samples were coated with gold using an Emscope 500 Sputter Coater (Ashford, Kent, Great Britain) as shown in Figure 3.13 to enhance conductivity. The Scanning Electron Microscope (SEM) JSM-6400 (Japan Electron Optics Laboratories, Joel, Japan) with a LaB6 emitter (Noran EDS) with analySIS Instructions software, as shown in Figure 3.14, was used to show the foamed images and calculate cell density and average cell size. a) Emscope Sputter Coater b) Sample c) Samples Figure 3.13 Emscope Sputter Coater a) Emscope, b) sample holder, and 0) samples. 70 6400 with a LaB6 emitter. Figure 3.14 The Scanning Electron Microscope (SEM): JSM 7l Determination of cell density and average cell size The SEM micrographs were used to calculate the number of cells by using one square inch Of nine images from the three areas of top, middle, and bottom. The number of cells was averaged and used to calculate cell density. The cell density per unit volume of the original unfoamed polymer was characterized from the scanning electron microscopy (SEM) micrographs using Equation 3.3 [10, ll], 3 an 5 1 No=( A ) [m] (3.3) where n is the number of cells, A is the area, and M is the magnification factor of the micrograph. The average cell size ((1) was calculated using Equation 3.4 [10, 1 l], 6VF d = 3 W (3'4) 00"" VF) Testing Mechanical properties An Instron (model 5565, Instron, Norwood, MA, USA) was used for measuring tensile strength, percent elongation at break, and tensile modulus following ASTM D 638. The test was carried out at room temperature with a 100 1b load cell at a constant crosshead speed Of 0.5 in/min, and with a grip separation of l in. For each condition, at least five samples were tested and the data averaged. 72 References l. 10. ll. Suh, K W., Handbook of Polymeric Foams and Foam Technology, Klempner, D and Sendijarevic, V., (ed), Hanser Publishers, Munich, p 198-199 (2004). Hello, S., Boyer, S. A. F., Padua, A. A. H., and Grolier, J. P. E., J. Polym. Sci. .- part B: Polymer Physics, 39, 2063-2070 (2001). Juntunen, R. P., Kumar, V., and Weller, J., J. Vinyl. Add. Tech, 6 (2), p 93-99, (2000) Hernandez, R., Selke, S. E., and Culter, J. D., Plastic Packaging: Properties, Processing, Applications, and Regulations, Hanser Publishers, Munich (2000). Tucker III, C. L., Injection and Compression Molding Fundamentals edited by Isayev, A. 1., Marcel Dekker Inc. Publisher, p 481-482 (1987). Doroudiani, S., Park, C. B., and Kortschot, M. T., J. Appl. Polym. Sci., 90, p 1412-1420 (2003). Hongliu, s. and James, E. M., J. Appl. Polym. Sci., 86, p 1692-1701 (2002). Naguib, H. E., Park, C. B., and Reichelt, N., J. Appl. Polym. Sci., 91 (4), p 2661- 2668(2004) Sun, X., Liu, H., and Li, G., J. Appl. Polym. Sci., 93, p 163-171 (2004). Rachtanapun, P., Microcellular Foam of Polymer Blends of HDPE/PP and Their Composites with Wood Fiber, Ph.D. Dissertation, School of Packaging, Michigan State University (2003). Matuana, L. M., Park, C. B., and Balatinecz, J., Cellular Plast., 32 (5), p 449-467 (1996) 73 Chapter 4 The Effect of Foaming Temperature on Characteristics and Tensile Properties of Biomax® Microcellular Foams The foamability of polymers can be affected by the nature of the polymer matrix (the interfacial energy and surface tension), the amount of sorption of gas into the polymer matrix, the rate of absorption and diffusion of gas into the cells in the polymer matrix, the degree of supersaturation, and also the processing conditions (saturation pressure, saturation time, foaming temperature, and foaming time) [1,2]. When the cells are nucleated, the foam density decreases as the available blowing agent molecules diffuse into the cells. The growth rate of the cell is limited by the diffusion rate and stiffness of the viscoelastic polymer/gas solution [3]. In general, cell growth is affected primarily by the time allowed for the cell to grow, the temperature of the system, the state of supersaturation, the hydrostatic pressure or stress applied to the polymer matrix, and the viscoelastic properties of the polymer/gas solution [4]. The mechanical properties of cellular foams are generally based on properties of the neat polymers such as crystallinity, molecular weight, crosslinking, and cellular morphology [5]. The density, cell density, average cell size, and the degree of the Openness Of the cells are the main parameters determining the foam properties. Mechanical properties such as tensile strength or tensile modulus are important for structural foams. When foams are stretched under applied load, they experience a shearing effect [6]. 74 The tensile strength characteristic of a polymer, or material, is proportional to the cross-sectional area of the specimen (the applied force per unit area). With increasing thickness, the area also increases as well. The stress-strain behavior of foams depends on the foam density, cell size, shape, orientation of cells, and closed-cell or open-cell structure. The tensile stress-strain properties are dependent on the uniformity of the cell size [7]. A large cell (larger than the average cell) acts as a stress concentrator where tearing will initiate. Guan et al. [8] studied the effect of foaming time, temperature, pressure, and foaming reagent content on thermal, mechanical and dynamic mechanical thermal (using dynamic mechanical thermal analysis (DMTA)) properties of polyethylene terephthalate (PET) microcellular foams. These foams were prepared With a general hydraulic press above PET’s crystallization temperature (Tc) and below Tm. Foaming time of 1 to 5 minutes, foaming pressures higher than 8 MPa, foaming temperature from 450-500 K (177-227°C), and a foaming agent content of 6-12% were investigated. Tensile strength and breaking extension increased at some experimental conditions, showing strengthening and toughening effects at the same time. The Tg, Tc, and Tm of microcellular PET foams decreased, but their crystallinity increased with increasing foaming time. Zhang et al. [9] studied the effects of molecular weight (MW) and foam density on mechanical properties during tension and at the break point. They found that increasing MW changed the tensile behavior Of HDPE from brittle fracture to ductile fracture. The break strain increased with increasing HDPE molecular weight and the 75 toughness of foams increased with normalized density (the foam density divided by the polymer density) and HDPE molecular weight. Hongliu et al. [10] investigated the preparation, characterization, and mechanical properties of some microcellular polysulfone foams. These microcellular foams were found to have average cell sizes in the range of 1-10 pm and cell densities on the order of 1010-1014 cell/cm3. The tensile modulus of these foams increased with the square of their relative densities, and the tensile strengths were proportional to the densities. Polymeric foaming is a complex process that involves delicate thermodynamic phenomena and kinetic material transport. Since cell nucleation and growth are the factors that predominantly govern the final foam structure and quality, a fundamental understanding of the underlying mechanism of these processes is indispensable to improve their regulation and to optimize different processing technologies utilized in the foam industry [1 1]. Although numerous research studies on microcellular foams have been published in the last two decades, only a few studies of biodegradable polymers were carried out. In this research, the effects of the foaming temperature on the density, foamed size, average cell size, cell population density, tensile strength, percent elongation, and tensile modulus Of Biomax® microcellular foam were studied. The experimental results for tensile strength and tensile modulus were compared with the simple rule of mixtures, and with Moore’s empirical square power law, respectively. 76 Materials and methods: Materials: Biomax® 4026 was provided from DuPont in pellet form. Glycerol was from IT. Baker (Phillipsburg, NJ, USA), and 99.5% CO2 gas from Linde Gas (Lansing, MI, USA). Preparing samples: The samples of Biomax® were prepared using the mini-extruder and injection molding system, (Micro 15, DSM Microcompounding Machine, DSM Research, Netherlands) into a dumbbell shape in the size of 3.2 inch x 0.21 inch x 1.59 mm (length x width x thickness). The processing conditions were temperature of 210°C in all sections, screw speed 100 rpm, melting time 2 min, and injection mold temperature of 23°C. Batch process Foaming temperatures of 100, 110,120, 130 and 140°C at a foaming time of 10 ® ® sec were used to prepared Biomax microcellular foams. After foaming, Biomax specimens changed from brown and translucent to white and Opaque, with smooth surfaces. All foamed samples were stored in air for at least two weeks before testing, to allow release of residual CO2. Statistical analysis Data were analyzed by a one-way ANOVA test using the SPSS (version 13) software program with a 95% confidence interval (9c=0.05) 77 Results and discussion Effect of foaming temperature on foamed dimensions ® All dimensions of Biomax microcellular foams increased as foaming temperature increased from 100 to 140°C, as shown in Table 4.1 and Figure 4.1. The thickness had a higher percentage increase, following by length and width. When the foaming temperature increased, the polymer chain stiffness decreased, causing the viscosity and surface tension of the polymer to decrease [5, 10] and with higher diffusivity, more C02 gas to diffuse fiom the polymer to the cells, leading to increased cell expansion and larger cell sizes [3, 12]. 78 Table 4.1 Dimensions of unfoamed Biomax® and foamed samples at different foaming temperatures and at 10 sec foaming time; a) length, b) width, and c) thickness. a) Length Samples Length % Increase (inch) Unfoamed BiomaxO 3,20 100°C 3.57:1: 0.20 11.48 110°C 4.10:1: 0.06 27.99 120°C 4.67i 0.28 45.82 130°C 5.193: 0.20 61.98 140°C 5.46:1: 0.15 70.46 b) Width Samples Width % Increase (inch) Unfoamed Biomax‘ 0,21 100°C 0.21:1: 0.01 3.79 110°C 0.25:1: 0.01 21.36 120°C 0.28:1: 0.02 36.53 130°C 0.31:1: 0.01 50.49 140°C 0.375: 0.01 78.31 c) Thickness Samples Thickness % Increase (mm) Unfoamed Biomfl 1,59 100°C 1.97:1: 0.08 23.97 110°C 2.14:1: 0.01 35.00 120°C 2.22d: 0.01 40.00 130°C 2.79:l:0.07 75.75 140°C 3.39i 0.07 113.54 79 Figure 4.1 Unfoamed Biomax® and foamed samples at 100, 110 120, and 130°C foaming temperatures (from left to right) at 10 sec foaming time. Effect of foaming temperature on foamed density, relative density and void fraction of Biomax® microcellular foams In addition as the cell expansion increased at the higher temperature, the cell wall thickness decreased and density decreased, as shown in Table 4.2 and Figure 4.2. Density exponentially decreased with increasing foaming temperature from 100-140°C (0.99 to 0.25 g/cm3). The R2 value for the correlation was 0.99. Microcellular foaming can reduce the material required to produce an item of a given size by 27 % (lower void fraction) to 81% (higher void fraction), which also reduces material cost. A one way ANOVA with 95% confidence interval (in Appendix A, Table A1) showed all densities were significantly different from each other except for 130 and 140°C foaming temperature. 80- Table 4.2 Density, relative density, and void fraction of unfoamed Biomax® and foamed samples at different foaming temperatures with 10 sec foaming time. Temperatures Density Relative Density Void (°C) (g/cm3) Fraction Unfoamed BiomaxE 1.35:1:0111 1,00 0 100 0.99i0.05 0.73 0.27 110 0.64i0.05 0.47 0.53 120 0.52:1:0.06 0.39 0.61 130 0.3321003 0.24 0.76 140 0.25i0.01 0.19 0.81 1.2 - 0.99 y ___ 29.277 €41.03“: R2 = 0.9897 0.8 - ”a U 39 .4? E- 0.4 - Q) Q 0 f I I I I VI 90 100 110 120 130 140 150 Foaming temperatures (°C) Figure 4.2 Foamed density at different foaming temperatures at foaming time of 10 sec. Effect of foaming temperature on cell population density and average cell size of Biomax® microcellular foams Cell population density of all Biomax® foams was in the range of microcellular foam (109-1015 cell/cm3), as shown in Table 4.3 and Figure 4.3. The cell population 81 density of the foams slightly changed from 100-140°C (1x1010 to 4x1010 cells/cm3). Statistical analysis was not done due to insufficient data. Table 4.3 Cell population density and average cell size of Biomax® at different foaming temperatures with foaming time of 10 sec. microcellular foams Temperatures No Average cell size (°C) (cells/cm3) (pm) 100 1.06E+l0 4.36 110 3.09E+10 4.42 120 3.10E+10 4.98 130 4.06E+10 5.67 140 3.61E+10 6.62 1.0E+14- 3.? g a ,.. .5. 5 ‘5 E — =1 5 8 4.06E+10 E v1-0E+" ' l.06E+10 3'09“” E / . + A 3.61E+10 3.10E+10 1.0mm . 4 . . . 95 105 115 125 135 145 Foaming temperatures (°C) Figure 4.3 Cell population density of Biomax® temperatures at 10 sec foaming time. microcellular foams at different foaming As mentioned before, the cell expansion increased with increasing foaming temperature (Table 4.3 and Figure 4.4). Average cell sizes of Biomax® foams were in the 82 range of microcellular foams (0.1-10 um), and exponentially changed with the foaming temperature, from 4.36 to 6.62 pm between 100 and 140°C. In a given foamed volume, gas will tend to diffuse from smaller bubbles into larger one; therefore, a fewer number of large cells is more stable than a large number of smaller cells [13]. 7 .- O 6.62 A 6 II E e 0 .5 {I} E 5 - C) g, 4.36 g; 9 y = 1.39786‘“""9x > < 4 - R’ =0.9456 3 r I I I I 95 105 115 125 135 145 Foaming temperatures (°C) Figure 4.4 Average cell size of Biomax® microcellular foams at different foaming temperatures at foaming time 10 sec. ® microcellular foams Effect of foaming temperature on morphology of Biomax SEM images Of Biomax® microcellular structures at different foaming temperatures from 100-140°C with a foaming time Of 10 see are shown in Figure 4.5. Biomax® microcellular foams are closed cell foams, as the cells are discrete from each other. At low temperature, 100°C, as shown in Figure 4.5 a, the cells started to foam as CO2 gas diffused into the cells, and the cells became spherical, but the polymer did not 83 foam uniformly. At this low temperature, the polymer matrix was still stiff, preventing the gas from diffusing rapidly, and cell walls were thick. As the temperature increased to l 10 °C, as shown in Figure 4.5 b, the cells expanded, became more uniform, and the average cell size increased. The increase of temperature decreased the Viscosity of polymer, reduced the force restricting the cell grth and provided faster gas diffusion (higher energy) into the cells, resulting in cell walls becoming thinner, but there were still some parts that did not foam well. At 120 and 130°C, as shown in Figure 4.5 c and d, the foams were uniform and well developed, and the cells changed from a spherical shape to polygons as the cells grew. At the highest foaming temperature of 140°C, shown in Figure 4.5 e, the cell size became even larger and some cells started to collapse, since the cell walls became too thin and the cell wall strength was not enough to resist the pressure in the cells. The foaming temperature must be between a value that provides a sufficiently low viscosity of the polymer matrix and lower polymer stiffness to allow CO2 gas to rapidly diffuse into the cells, and. a value that provides sufficiently high viscosity of the polymer to prevent cell collapse [14]. 84 c). 120°C ® Figure 4.5 SEM micrographs of Biomax microcellular foams at 10 sec foaming time at a magnification of 1000, at a) 100°C, b) l 10 °C, c) 120°C, (1) 130°C, and e) 140°C. 85 0 Effect of foaming temperature on tensile strength of Biomax microcellular foams Biomax® foams were brittle in tension, as shown in Figure 4.6. There was little or no macroscopic yielding prior to fracture, due to the inherent lack of tensile ductility [14]. Tensile strength of Biomax® microcellular foams decreased with increasing foaming temperature from 100 to 140°C (21.88 to 5.22 MPa), as shown in Table 4.4 and Figure 4.7. The brittleness in tension was probably caused by the stress concentrating effect of cracks nucleated at weak cell walls or pre-existing flaws propagating catastrophically, resulting in cell wall collapse and fracture [15]. When the density decreased, the cell walls became thinner. Therefore they were not strong enough to withstand the tension force. The statistical analysis using one way ANOVA with 95% confidence interval is shown in Appendix A, Table A2. A11 differences in tensile strength of Biomax® microcellular foams were statistically significant except those between 120 and 130°C foaming temperature with 10 sec foaming time. Tensile strength per unit volume (specific tensile strength) of foams showed different results from tensile strength. The highest specific tensile strength was about 27.3 MPa.cm3/g at 130°C. The lowest specific tensile strength was 20.8 MPa.cm3/g at 140°C, due to the cells starting to collapse. At low temperature (100°C), CO2 gas started to diffuse into the cells, so the cells expanded, strengthened and oriented until the cell walls were thinner (larger cell size) at 130°C, therefore the specific tensile strength was high at this point, before the cells started tO collapse at 140°C. 86 Table 4.4 Tensile strength, specific tensile strength, density, and relative density of unfoamed Biomax®and foamed samples at different foaming temperatures and at 10 sec foaming time. Samples Density Relative Tensile strength Specific tensile strength JQ/cm3) density (MPa) (MPa.cm3/g) Unfoamed Biomax® 1.35 1 36.001132 26.67 100°C 0.99 0.73 21.88il.50 22.10 110°C 0.64 0.47 15.65:h1 .24 24.45 120°C 0.52 0.39 11.101055 21.35 130°C 0.33 0.24 9.02:0.56 27.33 140°C 0.25 0.19 5.20:1:0.43 20.80 30 - 0 20 r o ’1? a. E :3, —0— unfoamed Biomax --— 100 C l- ‘v'5 1 104,, ---110C ---120C 1 ‘ -0- 130C - -- 140C 0 T T I I V 1 52 88 93 98 103 I08 Strain (%) Figure 4.6 Stress-strain curves of unfoamed Biomax® and foamed samples at different foaming temperatures at 10 sec foaming time. 87 40 -- -- 30 .1: ‘61: 5 30 -- e a A m \ 6°.“ 9 “a -r 20 = o E 3 a e 3 ea: up 0 5 20 -- g 1.. ., a 2 m g -- l0 1.. 10 c - l 'Tenslle strength ' ~ . l i Q l —0—Specific tensile strength ; i 0 1r t l t l 0 Unfoamed 100 110 120 I30 I40 Biomax Foaming temperature (°C) Figure 4.7 Tensile strength and specific tensile strength of unfoamed and foamed Biomax® at different foaming temperatures and a foaming time of 10 sec. Relationship between relative tensile strength and relative density The relative tensile strength was plotted as a function of relative density as shown in Table 4.5 and Figure 4.8. With an increase in relative density, relative tensile strength increased. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A3, indicates that relative density was significantly ® correlated to relative tensile strength of Biomax microcellular foams. In the case of tensile strength, the simple rule of mixtures predicts a linear relationship between the relative density and the relative tensile strength, as shown in Equation 4.1 [10], 88 O p . 0 m P». where Of and 0m are tensile stress at break of foamed and unfoamed polymers respectively, and pf and pm are the densities of the foamed and unfoamed polymers respectively. The experimental values of tensile strength are a generally good fit with the rule of mixtures, as shown in Figure 4.8. Table 4.5 Tensile strength, relative tensile strength, and relative density of unfoamed Biomax® and foamed samples at different foaming temperatures. Materials Tensile strength Relative tensile Relative density (MPa) strength [Unfoamed Biomax“ 36,00 1 1 100 °C 21.88 0.61 0.73 110 °C 15.65 0.43 0.47 120 °C 11.10 0.31 0.39 130 °C 9.02 0.25 0.24 140 °C 5.20 0.14 0.19 89 l i I: 0.8 -' ‘53 A E 2 3'3 0.6 r 2 "51‘ fl 3 g 0-4 " ‘ A Experiment data 3 — Theoretical results 32 0.2 . 0 I I I I I 0 0.2 0.4 0.6 0.8 1 Relative density Figure 4.8 Relative tensile strength as a function of relative density of Biomax® foams at different foaming temperatures at 10 sec foaming time. Effect of foaming temperature on percent elongation at break of Biomax“, microcellular foams As can be seen in Table 4.6 and Figure 4.9, elongation at break appeared to increase and then decrease with increasing foaming temperature. When force is applied to foam with a small cell size (low foaming temperature, 100°C), the thick cell walls can strengthen through orientation and elongate so percent elongation at break, can increase. However at larger cell sizes (higher temperature), the cell walls are thinner and consequently are not able to stretch as far without breaking, so percent elongation at break can decrease. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A4, indicates that differences in percent elongation at break were not statistically significant. There was high variation in tested 90 samples due to structural variability (not uniform in cell structure), so testing a large number of samples would be required to determine whether the apparent differences are significant. Table 4.6 Percent elongation at break of unfoamed Biomax® and foams at different foaming temperature at foaming time 10 sec. Temperatures Break elongation Standard deviation (°C) (%) Unfoamed-Biomax® 1203.65 132,24 100 47.34 15.87 110 79.24 18.42 120 83.94 19.93 130 66.19 4.65 140 63.09 8.88 90 l 84 80 p 79 8 .‘t 9:5 7° ‘ 66 .D ‘5 = .2 a 60 . 63 5 ii 50 . 47 40 . r . . . . 90 100 110 120 130 140 150 Foaming temperature (°C) Figure 4.9 Percent elongation at break of unfoamed Biomax®and foamed samples at different foaming temperatures at 10 sec foaming time. 91 0 Effect of foaming temperature on tensile modulus of Biomax microcellular foams As shown in Table 4.7 and Figure 4.10, tensile modulus of Biomax® microcellular foams decreased with increasing foaming temperatures, which was related to density. Tensile modulus decreased from 2.22 to 0.40 MPa (100 to 140°C foaming temperature). The specific tensile modulus (tensile modulus /density) of the foams slightly decreased when foaming temperatures increased. The specific tensile modulus decreased most between 100 and 120°C, with a slower decrease from 120 to 140°C. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A5, showed that differences in tensile modulus were statistically significant, except between 120 and 130 °C, and between 130 and 140 °C. Table 4.7 Tensile modulus, relative tensile modulus, and relative density of unfoamed Biomax® and foams as a function of foaming temperatures at 10 sec foaming time. Tensile Specific tensile Materials Density Relative modulus modulus (g/cm3) density (MPa) (MPa.cm3/g) Unfoamed Biomaxab 1.35 1 5.50:t0.56 4.07 100 °C 0.99 0.73 2.22:1:0.30 2.24 110 °C 0.64 0.47 1.31:l:0.l7 2.05 120 °C 0.52 0.39 0.951004 1.83 130 °C 0.33 0.24 0.553005 1.67 140 °C 0.25 0.19 0.40:1:0.14 1.60 92 ,_ 5.50 ’ w i l 6 - D “Tensile modulus E .. 4 :1 A —o—Specific tensile '2 ’53 a 5 I” modulus E "\ S 4 .4.» a ‘ o- u v 43 3 '5 n O m 2 H = a a e o- E i 2 I 67 1 60 2 m 0; : fi‘ 5 E- l' l 0.55 . 0.40 Unfoamed 100 110 120 130 I40 Biomax Foaming temperature (°C) Figure 4.10 Tensile modulus and specific tensile modulus of unfoamed Biomaxo and foams as a function of foaming temperature at foaming time 10 sec. Relationship between relative modulus and relative density Table 4.8 and Figure 4.11 show that with an increase in the relative density, the relative tensile modulus increased. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A6, shows that relative density ® was significantly correlated with relative tensile modulus Of Biomax microcellular foams. Moore’s empirical equation [10, 16-18] was used to predict the relationship between the relative modulus and the relative density using Equation 4.2, which is very close to a square power-law relationship: Er pr 2 E -( ) (4.2) where Ef and Em are modulus of foamed and unfoamed polymers respectively, pf and pm are the densities of the foamed and unfoamed polymers respectively. The experimental data for tensile modulus was compared with Moore’s empirical equation and showed a good relationship, as illustrated in Figure 4.11. Table 4.8 Tensile modulus, relative tensile modulus, and relative density of unfoamed Biomax® and foams as a function of the foaming temperature at foaming time 10 sec. Tensile Materials modulus Relative tensile Relative density (MPa) modulus Unfoamed Bioma? 5.50:1:056 1 1 100 °C 2.22:1:0.30 0.40 0.73 110 °C 1.3l:l:0.l7 0.24 0.47 120 °C 0.95:l:0.04 0.15 0.39 130 °C 0.55:1:0.05 0.10 0.24 140 °C 0.40:l:0.14 0.07 0.19 94 0.8 ‘ tn .2 = '6 c E 0.6 4 2 ‘5 a o «H o 0.4 . > 21:: .2 o o: 0.2 4 0 Experimental data — Theoretical results o d T I I I I 0 0.2 0.4 0.6 0.8 I Relative density Figure 4.11 Relative tensile modulus as a function of relative foam density of unfoamed Biomax® and foams at different foaming temperatures at 10 sec foaming time. 95 Conclusions In this experiment, Biomax® microcellular foams were successfully developed, yielding closed cell foams. The effects of foaming temperature on sample size, density, cell population density, average cell size, morphology, tensile strength, percent elongation at break, and tensile modulus were studied for the batch process using CO2 as a blowing agent. One way ANOVA with 95% confidence interval was used to analyze data. Biomax® microcellular foams were successfully produced in the range of 109-1015 cell/cm3 cell population density and 0.1-10 um cell size. When the foaming temperature increased, cell expansion increased, cell sizes were larger, and the cell wall thickness and density decreased [3, 15]. All dimensions of foams increased with increasing foaming temperatures, especially in thickness. ® The morphology studies of Biomax microcellular foams showed that the cells started to foam as CO2 gas diffused into the cells, and the cells became spherical at low temperature (100°C), when the polymer matrix was still stiff, retarding diffusing of the gas. As the foaming temperature increased (1 10-130°C), the cells expanded to polygons, the cell size increased, resulting in uniform cells and thinner cell walls. The increase of temperature decreased. the viscosity of the polymer, reduced the retraction force restricting the cell growth, and provided faster gas diffusion (higher energy) into the cells, resulting in cell walls becoming thinner until cell walls started to collapse at too high a temperature (140°C). . . . . ') . Biomax® microcellular foams were brittle but unfoamed BiomaxL8 was ductile. Tensile strength decreased with increasing foaming temperatures. Tensile strength Of 96 foams was related to density, at higher density, tensile strength was higher (more solid materials were available to bear the load), compared to lower densities. Experimental data for relative tensile strength was related to relative density as described by the rule of mixtures. ® Tensile modulus of Biomax microcellular foams decreased with increasing foaming temperatures. The experimental data for the relative tensile modulus as a function of the relative density was compared with Moore’s empirical equation, and showed a good relationship. 97 References 1. 10. 11. 12. 13. 14. Rachtanapun, P., Microcellular Foam of Polymer Blends of HDPE/PP and Their Composites with Wood Fiber, Ph.D. Dissertation, School of Packaging, Michigan State University (2003). Park, C. P., Polymeric Foams and Foam Technology, Klempner D., Sendijarevic V., (ed), Hanser Gardener publications, Cincinnati, Ohio, USA, p 267 (2004). Naguib, H. E., Park, C. B., and Reichelt, N., J. Appl. Polym. Sci., 91 (4), p 2661- 2668(2004) Baldwin, D. F., Park, and C. B., Suh, N. P., Polym. Eng. Sci., 38 (4), p 674-688 (1998) Doroudiani, S., Park C. B., and Kortschot, M. T., J. Appl. Polym. Sci., 90, p 1412- 1420 (2003). Throne, J., Sherwood Technology, “Shock Mitigation in Low-Density Thermoplastic Foams Part II”, http://www.foamandform.com/minutes/foamcolnpression2.php, (accessed April 15). Nielsen, L. E., and Landel, R. F., Mechanical Properties of Polymers and Composites, Marcel Dekker, Inc., New York, (1994). Guan, R., Wang, 3., and Lu, D., J. Appl. Polym. Sci., 88, p 1956-1962 (2003). Zhang, Y., Rodrigue. D., and Ati-Kadi, A., J. Appl. Polym. Sci., 90, p 2130-2138 (2003) Hongliu, S. and James, E. M., J. Appl. Polym. Sci., 86, p 1692-1701 (2002). Leung, S. N., Li, H., and Park, C. B., J. Appl. Polym. Sci., 104 (2), p 902-908 (2007) Sun, X., Liu, H., and Li, G., J. Appl. Polym. Sci., 93, p 163-171 (2004). Zhang, Y., Rodrigue, D., and Ait-kadi, A., J. Appl. Polym. Sci., 90, p 2111-2119 (2003). Goods, S. H., Neuschwanger, C. L., Henderson, C. C., and Shala, D. M., J. Appl. Polym. Sci., 68 (7), p 1045-1055 (1998). 98 15. Xing, F. and Park, 03., Polymeric Foams and Foam T echnologr, Klempner D., Sendijarevic V., (ed), Hanser Gardener publications, Cincinnati, Ohio, USA, p 324 (2004). 16. Moore, D. R. and Iremonger, M. J., J. Cell. Plast., 10, p 230-236 (1974). 17. Ozkul, M. H., Mark, J. E., and Aubert, J. H., J. Appl. Polym. Sci., 48, p 767-774 (1993) 18. Zhang, Y., Rodrigue, D., and Ati-Kadi, A., J. Appl. Polym. Sci., 90, p 2120-2129 (2003). 99 Chapter 5 The Effect of Foaming Time on Characteristics and Tensile Properties of Biomax® Microcellular Foams In this study, the effects of the foaming time on the density, foamed size, average cell size, cell population density, tensile strength, percent elongation, and tensile modulus ® of Biomax microcellular foam were studied. The experimental results for tensile strength and tensile modulus were compared with the simple rule of mixtures, and with Moore’s empirical square power law, respectively. Materials and methods Materials Biomax® 4026 was provided from DuPont in pellet form. Glycerol was from IT Baker (Phillipsburg, NJ, USA), and 99.5% CO2 gas from Linde Gas (Lansing, MI, USA). Preparing samples The samples of Biomax® were prepared using the mini-extruder and injection molding system, into a dumbbell shape in the size Of 3.2 inch x 0.21 inch x 1.59 mm (length x width x thickness). The processing conditions were temperature of 210°C in all sections, screw speed 100 rpm, melting time 2 min, and injection mold temperature of 23°C. Batch process Foaming temperatures Of 120°C at a foaming time Of 10, 20 and 30 sec were used to prepared Biomax® microcellular foams. After foaming, Biomax® specimens changed from brown and translucent to white and Opaque, with smooth surfaces. All foamed 100 samples were stored in air for at least two weeks before testing, to allow release of residual CO2. Results and discussion Effect of foaming time on foamed dimensions The dimensions of Biomax® foams increased as foaming time increased from 10 to 30 sec as shown in Table 5.1 and Figure 5.1. The percent increase was large between 10 sec and 20 see, but less between 20 and 30 sec. The cells at the foaming time of 20 sec had uniform cells and thin cell walls, while at the longer foaming time, 30 sec, there was a longer time for gas diffusion into the cells, so the cell walls became thinner and started to collapse. 101 Table 5.1 Dimensions of unfoamed Biomax® and foams different foaming times at 120°C foaming temperature, a) length, b) width, and 0) thickness. a) Length Samples Length % Increase (inch) Unfoamed Biomaxdis 3,20 120°C 10 sec 4.67:1: 0.28 45.82 120°C 20 sec 4.90:h 0.15 53.01 120°C 30 sec 5.063: 0.20 58.00 b) Width Samples Width % Increase (inch) Unfoamed Biomax® 0,21 120°C 10 sec 0.28:1: 0.02 36.55 120°C 20 sec 0.32:1: 0.01 53.98 120°C 30 sec 0.33:1: 0.01 60.05 c) Thickness Samples Thickness % Increase (mm) Unfoamed BiomaxB 1,59 120°C 10 sec 2.222t 0.01 40.00 120°C 20 sec 2.623: 0.09 65.06 120°C 30 sec 2.662h 0.11 67.55 102 Figure 5.1 Unfoamed Biomax® and foamed samples at the 120°C at 10, 20, and 30 see from left to right. Effect of foaming time on density of unfoamed Biomaxo and foams Table 5.2 and Figure 5.2 show that density decreased with increasing the foaming time from 10 to 30 sec (0.52 to 0.30 g/cm3). When the foaming time increased from 10- ® 30 sec, the relative density decreased from 0.39 to 0.22. Biomax microcellular foams can reduce the material required to produced an item of given size by 61 to 78%, as shown in Table 5.2. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A7, showed that all densities were significantly different from each other except those at 20 and 30 see, which had relative close values. Table 5.2 Density of unfoamed Biomax® and foams at different foaming times at 120°C foaming temperature. Foaming time Density Relative density Void (sec) (g/cm3) Fraction Unfoamed Biomax“ 13540.01 1.00 0.00 10 0.52i0.06 0.39 0.61 20 0.34:1:0.02 0.25 0.75 30 03010.02 0.22 0.78 103 1L8- 0.6 1 y = -0.2067Ln(x) + 0.9859 0.52 R2 = 0.9597 ”a 0 EB 0.4 - E‘ “J = Q :1 ILZ' 0 r I I I I fl 5 10 15 20 25 30 35 Foaming time (see) Figure 5.2 Density of Biomax® foams at different foaming times with foaming temperature of 120°C. Effect of foaming time on cell population density and average cell size of Biomax® microcellular foams Cell population density of all Biomax® foams was in the range of microcellular foam (109-10l5 cell/cm3), as shown in Table 5.3 and Figure 5.3. Cell population density of the foam slightly decreased as foaming time increased from 10 to 30 sec, going from 3.1x1010 to 8.6x109 cells/cm3. Statistical analysis was not performed due to insufficient data. 104 Table 5.3 Cell population density and average cell size of Biomax® foams at different foaming times with 120°C foaming temperature. Time No Average cell size (see) (cells/cm3) (um) 10 3.10E+10 4.98 20 1.54E+10 7.71 30 8.60E+09 ' 9.86 l.0E+12 - E “E 1.0a+11- «g a; 3.1B+10 El. E l.5E+10 a V g 8.6E+09 O l.0E+10 ‘ 1.0r:+09 . . . . . fl 5 10 15 20 25 30 35 Foaming time (see) Figure 5.3 Cell population density of Biomax® foams at the different foaming times with foaming temperature of 120°C. As shown in Table 5.3 and Figure 5.4, average cell size of Biomax® foams increased when the foaming time increased from 4.98 to 9.86 pm (10 to 30 sec foaming times) with R2 =0.99. The CO2 gas had a longer time to diffuse into cells from 10 to 20 sec until at 30 see the cells started to collapse. Statistical analysis was not performed due to insufficient data. 105 II- 936 y = 0.2m + 2.6369 4.98 2 R = 0.9955 Average cell size (um) \I 3 T I I I I I 5 10 15 20 25 30 35 Foaming time (sec) Figure 5.4 Average cell size of Biomax® foams at different foaming times with 120°C foaming temperature. @. Effect of foaming time on morphology of Biomax microcellular foams Figure 5.5 shows SEM images of Biomax® microcellular structures at the different foaming times from 10 to 30 sec, at a saturation pressure of 850 psi and a foaming temperature of 120°C. These foams are closed cell foams, as the cells are discrete from each other. As shown in Figure 5.5 a, at a foaming time of 10 sec, the C02 gas dispersed into the saturated polymer forming spherical cells, and the cellular structures grew but the material still had some parts that did not foam. When the foaming time increased to 20 sec (with longer C02 diffusion into the cells), as shown in Figure 5.5 b, the cellular structures of the foam were more uniform, the cell size was bigger, and the cell walls became thinner. Figure 5.5 c shows that at the foaming time of 30 sec (the 106 longest time), the cell size became bigger and started to collapse since the cell walls became too thin and the strength was not enough to resist the pressure in the cells. c) 30 sec 20 “m Figure 5.5 SEM micrographs of Biomax® microcellular foam at the different foaming times for 120°C foaming temperature at magnification of 1000, a) 10 sec, b) 20 sec, and c) 30 sec. 107 Effect of foaming time on tensile strength of Biomax microcellular foams The stress-strain curves, as shown in Figure 5.6, indicated Biomax® foams were brittle, but on the other hand unfoamed Biomax® was ductile. Tensile strength of Biomax® microcellular foams decreased with increasing foaming times at the foaming temperature of 120°C (with 850 psi saturation pressure and 2 days saturation time) as shown in Table 5.4, Figure 5.6, and Figure 5.7. The highest tensile strength was 36 MPa for unfoamed Biomax®, and the lowest was 7.8 MPa at the foaming time 30 sec. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A8, showed significant differences of tensile strength at each foaming time. As shown in Table 5.4, tensile strength of Biomax® microcellular foams depended on relative density. The highest tensile strength was 36 MPa for unfoamed Biomax®, and the lowest was 7.8 MPa at the lowest relative density of 0.22. However, the specific tensile strength was highest, 29.4 MPa.cm3 / g, at 0.25 relative density, which was higher than unfoamed Biomax®, and was lowest, 21.4 MPa.cm3/g, at 10 sec. The orientation of cells at 10 sec was less than at 20 sec (larger cell size) so the 20 sec foaming time had higher specific tensile strength. At 30 sec foaming time, the cells started to collapse, making weak spots in the cells; therefore, specific tensile strength decreased. 108 Table 5.4 Tensile strength and specific tensile strength of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C. Samples Density Relative Tensile Strength Specific tensile strength (g/cm’) density (MPa) (MPa.cm3/g) Unfoamed Biomax® 1.35 1.00 36.00 26.67 10 sec 0.52 0.39 11.10 21.35 20 sec 0.34 0.25 10.00 29.41 30 sec 0.30 0.22 7.79 25.97 40 . 30 4 20 Stress (MPa) 10 - t '20 sec —O- Unfoamed Biomax - I - 10 sec - iii '30 sec 100 200 300 Strain (%) 400 500 Figure 5.6 Stress-strain curves of Biomax® and foams at different foaming times at 120°C foaming temperature. 109 40 -- A H 40 - l- Tensile strength i 36.0 -'0— Specific tensile strength . , ‘ 3 5 g 30 -- -- 30 .- 9. G- g E 4: '5 en in § 2 20 -- -- 20 a ‘5 a 2 2 '5 2 5 a "‘ a '0 I p " " 1 0 =- U: 0 i l i 0 Unfoamed Biomax 10 20 30 Foaming time (sec) Figure 5.7 Tensile strength and specific tensile strength of unfoamed Biomax®and foams at different foaming times at foaming temperature 120°C. Relationship between relative tensile strength and relative density Relative tensile strength was plotted as the function of relative density of unfoamed Biomax® and foams at different foaming times at 120°C foaming temperature. As shown in Table 5.5 and Figure 5.8, with an increase in the relative density, tensile strength increased. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A9, indicated that relative density significantly affected relative tensile strength. In the case of tensile strength, the simple rule of mixtures predicts a linear relationship between the relative density and the relative tensile strength, as shown in 110 Equation 4.1 [1]. It was difficult to evaluate how well this relationship held, as few data points were available. Table 5.5 Tensile strength, relative tensile strength, and relative density of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C. Tensile Relative tensile Materials strength strength Relative density LMPQ) Unfoamed Biomaxdb 36.00:.092 1 l 10 sec 11.103055 0.31 0.39 20 sec 10.001033 0.28 0.25 30 sec 7.79:0.83 0.22 0.22 1 1 0.8 ~ .fl 8'» E g 0.6 ‘ E t“: 8 0.4 . g A Experimental data '5 A . g — Theoretical results 0 =2 0.2 . 0 f . . r . 0 0.2 0.4 0.6 0.8 1 Relative density Figure 5.8 Relative tensile strength as a function of relative density. Effect of foaming time on percent elongation at break of Biomax® microcellular foams As can be seen in Table 5.6 and Figure 5.9, percent elongation at break appeared to decrease as foaming time increased. At the shorter time of 10 see, there was less gas in 111 the cell, so the cell walls were still thick. At the longer foaming time of 20 sec, more C02 diffused into the cells, producing uniform cells. The cell walls were too thin and started to collapse at 30 sec foaming time. However, the statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A10, showed the differences in percent elongation at break as a function of foaming time, were not statistically significant. Table 5.6 Percent elongation at break of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C. Samples Break Elongation STD EV (%) Unfoamed Biomax® 1203.65 132,24 10 sec 83.94 19.93 20 sec 70.59 7.62 30 sec 36.18 10.25 112 71 704 Elongation at break (%) 30 i j I I t I 10 15 20 25 30 35 'Jl Foaming time (see) Figure 5.9 Percent elongation at break of Biomax® foams at different foaming times at foaming temperature 120°C. (D Effect of foaming time on tensile modulus of Biomax microcellular foams Measurement of the tensile modulus of Biomax® microcellular foams showed stiffness decreased as foaming temperature increased, as shown in Table 5.7 and Figure 5.10. The specific tensile modulus of foams appeared to slightly decrease when relative density increased. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A11, indicated no significant difference of foaming time on tensile modulus, except for the comparison of Biomax® with foams. The tensile modulus values of the foams are close to each other and the structural variation in the foamed samples (variation in cell sizes), means more samples are required for testing to determine if there are differences. 113 Table 5.7 Tensile modulus, specific tensile modulus, density, and relative density of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C. Specific tensile Samples Density Relative Tensile modulus modulus (g/cm3) density (MPa) (MPamflg) Unfoamed Biomax® 1.35 1.00 5.50m45 4.07 10 sec 0.52 0.39 0.95:l:0.04 1.83 20 sec 0.34 0.25 0.61:1:0.05 1.79 30 sec 0.30 0.22 0.47d:0.05 1.57 7 1' 5 5.5 . ._- ___m m 6 .. - D °Modulus 3 —0—Specific modulus +4 '§ A El 4 +~ 3 33: g 5 25 g 3 -+ 1.79 g" g 3 1.57 “2 W 1 4.. s. - - 0.61 0.47 I 0.95 - ' ' ' ' ------ . o .L 4 . o Unfoamed Biomax I0 20 30 Foaming time (see) Figure 5.10 Tensile modulus and specific tensile modulus of unfoamed Biomax®and foams at different foaming times at 120°C foaming temperature. Relationship between relative tensile modulus and relative density Table 5.8 and Figure 5.11 show the relationship between the experimental data for the relative tensile modulus and the relative density with Moore’s empirical equation [1- 4], Equation 4.2. The statistical analysis using one way ANOVA with 95% confidence 114 interval, as shown in Appendix A, Table A12, showed relative density significantly affected relative tensile modulus. It was difficult to evaluate how well the model fit the data, as few data points were available. Table 5.8 Modulus, relative modulus and relative densities of unfoamed Biomax® and foams at different foaming times at foaming temperature 120°C. Materials Tensile modulus Relative tensile Relative density (MPa) modulus Unfoamed Biomax® 5,50¢0,56 1 1 10 sec 0.95:l:0.04 0.17 0.39 20 sec 0.6ld:0.05 0.11 0.25 30 sec 0.47:l:0.05 0.09 0.22 1.0 ~ 2 0.8 i = '5 G E 0.6 . .2 '53 = 8 q, 0.4 l .2 E A Experimental data 0 a: 0.2 ‘ —' Theoretical results 0.0 " I I I I I 0 0.2 0.4 0.6 0.8 1 Relative density Figure 5.1 1 Relative tensile modulus as a function of the relative density. 115 Relationship between average cell sizes and densities at different foaming temperatures and foaming times The microstructures of the foamed samples were controlled through choice of the foaming temperatures and times, resulting in a wide range of foam densities and average cell sizes. Table 5.9 and Figure 5.12 show average cell size of foams as a function of density. As can be seen, average cell size was independent of density at density below 0.5 g/cm3. Although foams have the same density, they can have different average cell sizes, depending on the foaming process variables such as the foaming temperature and time, which are the main parameters controlling the density and average cell size of foams. The foaming temperature of 120°C at 20 sec resulted in foam with a density of 0.34 g/cm3 and an average cell size of 7.71 pm. In the case of a foaming temperature of 130 °C at 10 see, the density was 0.33 g/cm3, and the average cell size was only 5.67 um. Average cell size slightly decreased when the foamed density was 0.5 g/cm3 or above. Table 5.9 Density and average cell size at different foaming temperatures and times. Temperature (°C) Time (sec) Density Average cell size (glem’) (um) 120 20 0.34 7.71 120 30 0.30 9.86 100 10 0.99 4.36 110 10 0.64 4.42 120 10 0.52 4.98 130 10 0.33 5.67 140 10 0.25 6.62 116 12- . 9.86 9 I ”g o 7.71 3 S 6 . 4 98 E 6‘62 . . 4.36 5 5.67 o 4.42 . 31 0 I I I I I I I I I I 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 I .1 Density (g/cm’) Figure 5.12 Average cell size as a function of density of Biomax® foams at different foaming temperatures and foaming times. 117 Conclusions ® In this experiment, Biomax microcellular foams were successfully developed, yielding closed cell foams. The effects of foaming time on size, density, cell population density, average cell size, morphology, tensile strength, percent elongation at break, and tensile modulus were studied for the batch process using C02 as a blowing agent. One way ANOVA with 95% confidence interval was used to analyze data. When increasing foaming times, all dimensions of foams increased, especially in thickness; foamed density exponentially decreased; and cell population density slightly decreased. The morphology studies of Biomax® microcellular foams showed that at short foaming time (10 see), there was less C02 gas in the cells so the cells were small. When the foaming time increased (20 see), there was longer C02 diffusion into the cells, the cellular structures of the foam were more uniform, the cell size became bigger, and the cell walls became thinner, until the cell size became bigger and started to collapse at too long a foaming time (30 sec). ® Biomax microcellular foams were brittle but unfoamed Biomax® was ductile. Tensile strength decreased with increasing foaming time. Tensile strength of foams was related to density, at higher density, tensile strength was higher (more solid materials were available to bear the load), compared to lower densities. Foaming time significantly affected the tensile strength of Biomax® microcellular foams except at 10 and 20 sec foaming time with 120°C foaming temperature. Tensile modulus of Biomax® microcellular foams decreased with increasing foaming time. 118 References l. Hongliu, S. and James, E. M., J. Appl. Polym. Sci., 86, p 1692-1701 (2002). 2. Moore, D. R. and Iremonger, M. J., J. Cell. Plast., 10 (5), p 230-236 (1974). 3. Ozkul, M. H., Mark, J. 15., and Aubert, J. H., J. Appl. Polym. Sci., 48, p 767-774 (1993). 4. Zhang, Y., Rodrigue. D., and Ati-Kadi, A., J. Appl. Polym. Sci., 90, p 2120—2129 (2003) 119 Chapter 6 Effect of Thickness and Aging on Tensile Properties of Unfoamed and Foamed Biomax® Microcellular foams can be developed in a batch process. By controlling the foaming conditions (saturation pressure, saturation time, foaming temperature, foaming time) [1,2], a wide variety of densities and cell sizes can be produced [2]. Foaming generally reduces the mechanical properties of polymers. The mechanical properties of cellular foams are generally based on properties of the neat polymers such as crystallinity, molecular weight, crosslinking, and cellular morphology [1, 3], as well as processing conditions. Processing parameters such as foaming conditions (saturation time, saturation pressure, foaming temperature, and foaming time), solubility and diffusivity of gas, crystallinity, crystalline morphology can affect the foamability of materials and are related to the cellular structure and mechanical properties of the foams [1, 4]. Density, cell density, average cell size, and the degree of the openness of the cells are the other main parameters determining the foam properties. Physical aging, a common phenomenon, is characteristic of the glassy state of all materials. This process is therrnoreversible, which means that although a material has been physically aged, this aging can be removed by simply heating the material in excess of its T8. The aging typically occurs in the glass state as a consequence of room temperature storage, develops at a faster rate as the aging temperature (Ta) approaches Tg, and can be attributed to relaxation of the molecules toward equilibrium. Physical aging has a spectacular influence on polymer properties such as reducing impact strength 120 because it increases the relaxation times of the polymer, and can also result in the plasticizer migration and loss [5]. Environmental aging of cellular polymers is important in many applications, and especially in biodegradable polymer foams. Expansion of a cellular state increases the surface area; reactions of foam with vapor and liquids are correspondingly faster than with the solid polymer. All cellular polymers deteriorate under the combined effects of light or heat and 02 [6]. Kuroda and et al. studied tensile properties (ISO 527) of polymethyl methacrylate (PMMA) and polyvinyl chloride (PVC), finding an increase in the tensile strength after storage in a dark room for several years at 20°C and 65 % RH. In this case, where the polymers were stored for a long time at isothermal conditions, strengthening of the polymer structure was predominant rather than deterioration [7]. In this study, first the effects of the foaming time and temperature on size, ® density, cell population density, and average cell size of Biomax microcellular foams produced for samples with a rectangular shape; 2.29 inch x 0.49 inch x 2.12 mm (length x width x thickness) were evaluated. Next, these results were compared to those for the dumbbell shape (thickness 1.59 mm) discuss in Chapter 4, to evaluate the effects of thickness (identical shapes differing only in thickness could not be used as appropriate molds were not available). Last, after one year of aging, tensile strength, percent elongation and tensile modulus of unfoamed Biomax® and Biomax® foams at foaming temperatures of 120 and 130°C and 10 sec foaming time were compared to those previously obtained (Chapter 4) to evaluate the effects of aging. 121 Materials and methods Materials Biomax® 4026 was provided from DuPont in pellet form. Glycerol was from IT Baker (Phillipsburg, NJ, USA), and 99.5% C02 gas from Linde Gas (Lansing, MI, USA). Preparing samples The samples of Biomax® were prepared using the mini—extruder with injection molding system into a rectangular shape in thesize of 2.29 inch x 0.49 inch x 2.12 mm (length x width x thickness). The processing conditions were temperature of 210°C in all sections, screw speed 100 rpm, melting time 2 min, and injection mold temperature of 23°C . Batch process At the foaming temperature of 120° C, for times of 10, and 15 sec were used. At the foaming temperatures of 130 and 140°C, times of 5 and 10 sec were used. At a foaming temperature of 150°C, a time of 5 sec was used. After foaming, Biomax® specimens changed from brown and translucent to white and opaque, with a smooth surface. All foamed samples were stored in air for at least two weeks before testing, to allow release of residual C02. 122 Results and discussion Effect of foaming temperatures and times on foamed dimensions As can be seen in Table 6.1 and Figure 6.1 a, at a foaming time of 10 sec, all dimensions of the foams increased. The percent increase was from 11 to 33% in length, 8-30% in width, and 32—82% in thickness, as temperature increased from 120-140°C. For a foaming time of 5 see, as shown in Figure 6.1 b, the percent increase was from 7 to 24% in length, 5-24% in width, and 35-78% in thickness as temperature increased from 130-150°C. Thickness (2.12 mm) had the highest percent increase in foams. The foaming temperature and time affected the size of foams in the same way as described in the previous chapter. Table 6.1 Dimensions of unfoamed Biomax® and foams at different foaming temperatures and times. Samples Dimensions (foaming temp, Length % Width % Thickness % Foamingflne ) (inch) Increase (inch) Increase (mm) Increase Unfoamed Biomax 2,29 0,49 2,12 130°C, 5 sec 2.44 6.77 0.51 5.44 2.86 34.97 140°C, 5 sec 2.59 13.24 0.55 12.85 2.99 41.08 150°C, 5 sec 2.83 23.93 0.60 24.04 3.78 78.20 120°C, 10 sec 2.55 11.63 0.52 8.04 2.79 31.62 130°C, 10 sec 2.90 27.00 0.59 21.69 3.25 53.05 140°C, 10 sec 3.04 32.75 0.63 30.82 3.87 82.40 120°C, 15 sec 2.94 28.70 0.60 23.03 3.16 48.86 123 Unimed 720°C iso'c Unioml iso‘c l40'C iso‘c a) Foaming time of 10 sec b) Foaming time of 5 sec Figure 6.1 Effect of foaming temperatures on size of Biomax® different foaming times: a) 10 sec and b) 5 sec. microcellular foams at As can be seen in Table 6.1 and Figure 6.2 a, b, and c, all foamed samples became bigger with higher foaming time. All dimensions increased from the lower foaming times to the higher. The highest percent increase was in the thickness. 124 a) 120°C b) 130°C c) 140 °C Figure 6.2 Effect of foaming times on the size of Biomax® microcellular foams at the different foaming temperatures a) 120°C at 10 and 15 sec, b) 130°C at 5 and 10 sec, and c) 140°C at 5 and 10 sec. Effect of foaming temperature and foaming time on foamed densities The relationship between density and foaming conditions (foaming temperature, and foaming time) of Biomax® microcellular foams was discussed in chapter 4. Foaming temperature and foaming time affected the density as shown in Table 6.2 and Figure 6.3. The statistical analysis using one way ANOVA, with a 95 % confidence interval, as shown in Appendix A, Table A 13, showed that foaming temperature significantly 125 affected density at all foaming temperatures and foaming times, but there was no correlation between temperature and time. The results shown in Table 6.2 indicate that Biomax® microcellular foams decreased material use per unit volume by 23-61%. Table 6.2 Results of density, relative density, and void fraction of unfoamed Biomax®, and foams at different foaming temperatures and times. Samples Density Relative Density Void Fraction Temp (°C) Time (sec) (g/cm3) Unfoamed Biomax® 134:0,01 1 0 130 5 1.03:1:0.03 0.77 0.23 140 5 0.84:1:0.09 0.63 0.37 150 5 0.67:l:0.04 0.50 0.50 120 10 0.89:1:0.09 0.66 0.34 130 10 0.66:1:0.06 0.49 0.51 140 10 0.52:1:0.03 0.39 0.61 120 15 0.61:h0.02 0.46 0.54 Table 6.3 Comparison of percent decrease of density at foaming times (5, 10 sec) at different foaming temperatures. Samples Density % Decrease Temperature Time (g/cm3) (°C) (5801 130 5 1.03 140 5 0.84 18.45 150 5 0.67 34.95 120 10 0.89 130 10 0.66 25.84 140 10 0.52 41.57 126 Table 6.4 Comparison of percent decrease of density at foaming temperatures (120, 130, and 140°C) at different foaming times. Samples Density % Decrease Temperature Time (g/cm3) (°C) (see) 120 10 0.89 120 15 0.61 31.46 130 5 1.03 130 10 0.66 35.92 140 5 0.84 140 10 0.52 38.10 Density Unfoamed I30. 5 I40. 5 150. 5 120. 10 130, 10 I40, 10 120. IS Biomax Temperature, time (C, see) Figure 6.3 Density of Biomax® and times microcellular foams at different foaming temperatures 127 Effect of foaming temperature and foaming time on cell population density and average cell size of foams Cell population densities of foams were in the range of microcellular foam. The cell density changed slightly from 7.5><109 to 1.9x1010 cells/cm3 when the foaming temperature and foaming time increased, as shown in Table 6.5 and Figure 6.4. Statistical analysis was not done due to insufficient data. Table 6.5 Results of cell population density and average cell size of unfoamed Biomax® and foams at different foaming temperatures and times. Sam les No Average cell size Temp (°C) Time (sec) (cells/cm3) (pm) 130 5 7.50E+09 4.57 140 5 1.55E+10 4.70 150 ' 5 1.79E+10 5.11 120 10 1.44E+10 4.40 130 10 1.92E+10 5.06 140 10 1.27E+10 6.64 120 15 1.15E+10 6.26 128 1.E+15 - LIE-HZ - 7.5E+09 LIE-+09 ‘ Cell density (cells/cm") LEW} . I30. 5 I40, 5 Figure 6.4 Cell population density of Biomax temperatures and times. l.6F.+10 l.8E+10 150,5 ”5*” 135*” 1.31310 1 115+"; 120,10 130,10 140,10 120,15 Temperature, time (°C, sec) ® microcellular foams at different foaming Table 6.5 and Figure 6.5 show that average cell sizes of foams increased with increasing foaming temperatures and foaming time. The cell size increased more with increase in foaming time (5 to 10 sec) than increasing temperature by 10°C, as Shown in Table 6.6 and 6.7. Statistical analysis was not done due to insufficient data. Table 6.6 Comparison ofpercent increase of average cell size at foaming times (5, 10 sec) at different foaming temperatures. Temperature Time Average cell size % Increase (°C) (sec) (f' m) 130 5 4.57 140 5 4.51 -1.28 150 5 5.11 1 1.78 120 10 4.40 130 10 5.06 15.00 140 10 6.64 50.80 129 Table 6.7 Comparison of percent increase of density at foaming temperatures (120, I30, and 140°C) at different foaming times. Temperature Time Average cell size % Increase (°C) (see) 0”") 120 10 4.40 120 15 6.26 42.15 130 5 4.57 130 10 5.06 10.70 Average cell size (um) 130,5 140,5 150,5 120,10 130,10 140,10 120,15 Temperature, time (°C, sec) ® Figure 6.5 Average cell sizes of Biomax microcellular and foams at different foaming temperatures and times. Effect of foaming temperature and time on foamed morphology ® SEM images of Biomax microcellular foams are shown in Figure 6.6 through Figure 6.12 at different foaming temperatures and times, with a magnification of 1000. ® Biomax microcellular foams were closed cell, as the cells were discrete. The microcellular foams developed differently in the samples. The micrographs showed the 130 foams developed more towards the outside than the middle (centerline). Figure 6.9 (130°C foaming temperature, 5 sec foaming time) shows that the inside of the foam samples did not foam due to the low foaming temperature and too short a foaming time, even though the outside cells started to develop into a small spherical shape. This behavior was due to the intrinsically high viscosity and elasticity, which restricted the expansion of the cells. At 140°C and 5 see, as shown in Figure 6.11, the outside areas foamed better than the inside and developed more cells than at 130°C foaming temperature. At higher foaming temperature and time as in Figures 6.8, 6.10, and 6.13, uniform foamed samples were developed, with larger cell sizes and thinner cell walls, but the middle of the samples still did not foam well, with thicker cell walls and smaller cell sizes than the outside. When the foaming temperature was 140°C with the foaming time at 10 see, as shown in Figure 6.12, the foamed samples were homogeneous, independent of the depth from the surface, with thinner wall cells and well developed cells, implying sufficient foaming temperature and time. In this experiment, we did not foam at higher foaming temperatures than 150°C, or for longer than 15 sec foaming time, due to warping of the foams when they were placed into the hot glycerol bath, which was difficult to control. 131 Middle ' 20 um Figure 6.6 SEM images of 120°C foaming temperature at 10 sec foaming time at magnification 1000 132 Middle Figure 6.7 SEM images of 120°C foaming temperature at 15 sec foaming time at magnification 1000 I33 Top Left ‘ 1 Middle Right 20“, Figure 6.8 SEM images of 130°C foaming temperature at 5 sec foaming time at magnification 1000 134 Figure 6.9 SEM images of 130°C foaming temperature at 10 sec foaming time at magnification 1000 135 Middle 20 um Figure 6.10 SEM images of 140"C foaming temperature at 5 sec foaming time at magnification 1000 136 20 um Figure 6.11 SEM images of 140°C foaming temperature at 10 sec foaming time at magnification 1000 137 Middle Right 20 um Figure 6.12 SEM images of 150°C foaming temperature at 5 sec foaming time at magnification 1000 138 Relationship between average cell sizes and densities The same average cell size of foamed samples can result in a wide range of densities through varying the foaming temperatures and times. Table 6.8 and Figure 6.13 show the relationship between average cell size of the foams and density. At low density (below 0.8 g/cm3),when density increased, the average cell size decreased. Average cell size slightly changed when foamed density was 0.8 g/cm3 or above. Table 6.8 Densities and average cell sizes at different foaming temperatures and times. Temperature Time Density Average cell size (°C) (sec) (M3) (pm) 120 10 0.89i0.09 4.40 120 15 0.61:1:0.02 6.26 130 5 1.03:t:0.03 4.57 130 10 0.66:|:0.06 5.06 140 5 0.84d:0.09 4.51 140 10 0.52:I:0.03 6.64 150 5 0.67d:0.04 5.11 139 Average cell size (um) 1.2 O 6.64 O 6.26 6 4 5.11 5 . fl 5-06 4.51 O 4 S7 ’ o 4 'i 4.40 3 I I I f f 1 I I 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 Density (g/cm’) Figure 6.13 Average cell sizes as a function of the relative densities 140 The Effect of Thickness of Dumbbell Shape (1.59 mm) and Rectangular Shape (2.12 mm) on Size, Density, Cell Population Density, and Average Cell size of Biomax Microcellular Foams Materials and methods The results of foaming temperature 120 and 130°C with 10 sec foaming time in Chapter 4 for the dumbbell shape (1.59 mm thickness) and the rectangular shape (2. 12 mm thickness) from the first part of chapter 6 were compared. Results and discussion Effect of sample thickness on foamed dimensions As shown in Table 6.9, the sizes of both dumbbell and rectangular shapes increased with increasing foaming temperatures in all dimensions. The dumbbell shape (thinner) dimensions increased more than the rectangular shape, especially in thickness. The microcellular foams were developed differentially through the thickness due to the temperature gradient (higher at outside surface through inside sample). The outside surface developed foams faster than the centerline; the thicker the samples, the lower the temperature in the middle of cells. This requires higher foaming temperature to develop foams. 141 Table 6.9 Comparison of dimensions for thinner dumbbell and thicker rectangular shapes, at different temperatures at 10 sec foaming time a) length, b) width, and 0) thickness. a) Length Samples Dumbbell shape Rectangular shape Length (in) % Increase Length (in) % Increase Unfoamed Biomax‘E 3.202 0.00 2.29i 0.00 120°C 4.67:l: 0.28 45.82 2.55:1:0.03 11.63 130°C 5.191: 0.20 61.98 2.90:l:0.06 27.00 140°C 5.46i 0.15 70.46 3.04fl.04 32.75 b) Width Samples Dumbbell shape Rectmular shape Width (in) °/o Increase Width (in) % Increase Unfoamed BiomaxJ 0.21=e 0.00 0.493: 0.00 120°C 0.28:I: 0.02 36.53 0.52t0.01 8.04 130°C 0.31:1: 0.01 50.49 0.59d:0.01 21.69 140°C 0.373: 0.01 78.31 0.63:1:0.02 30.82 0) Thickness Samples Dumbbell shape Rectangular shape Thickness % Thickness "/o (mm) Increase (mm) Increase Unfoamed Biomax® 1,591 0.00 2.12:1: 0.00 120°C 2.22i 0.01 40.00 2.79t0.07 31.62 130°C 2.79i0.07 75.75 3.25:1:0.06 53.05 140°C 3.39:1: 0.07 113.54 3.87:|:0.15 82.40 Note: Weight of dumbbell specimen was about 1.32 g Weight of rectangular specimen was about 1.96 g 142 Effect of thickness on foamed densities Densities of the dumbbell shape and rectangular shape foams decreased with increasing foaming temperatures, as shown in Table 6.10 and Figure 6.14. At the same conditions (foaming temperature and time), the dumbbell shape had lower density than the rectangular shape, as with faster heating, more cells developed from the surface through the center of samples. Density of the thinner dumbbell shape can be as low as 0.25 g/cm3 compared to 0.52 g/cm3 in the thicker rectangular shape at the foaming temperature of 140°C with 10 sec foaming time. One way ANOVA with 95% confidence interval (as seen in Appendix A, Table A14) showed that the densities of the dumbbell shape were significantly different fi'om those of the rectangular samples at equal foaming temperatures and time. Table 6.10 Comparison of density between thinner dumbbell and thicker rectangular shapes at different foaming temperatures at 10 sec foaming time. Temperature Density (°C) Dumbbell Rectangular 120 0.52:1:0.06 0.89:1:0.09 130 0.33:l:0.03 0.66t0.06 140 0.25:1:0.01 0.52=I=0.03 143 g 0-89 y = -0.0185x + 3.095 “ . Rz=0.9807 0.8 - ~ . ~ ORectangular ~ ~ ~ IDumbbell Q ~ ~ ~ £1 ME 0.6 0.66 ~ . to ~ 5 g, . a v " 0.52 0.4 . 0.52 o 2 , _. = -0.0135x + 2.1217 ' 0.25 R2 = 0.9476 0 r I ‘r r I I 115 120 125 130 135 140 145 Temperatures (°C) Figure 6.14 Comparison of density between thinner dumbbell and thicker rectangular shapes at different foaming temperatures at 10 sec foaming time Effect of thickness on cell population density of foams From Table 6.11 and Figure 6.15, cell population density of both dumbbell and rectangular shapes changed only slightly when foaming temperature increased. The cell population density of the thinner dumbbell shape specimens was consistently higher than the cell density of the thicker rectangular shape. Table 6.11 Comparison of cell p0pulation density between thinner dumbbell and thicker rectangular shapes at different foaming temperatures at 10 sec foaming time. Temperature N0 (cells/cm3) (°C) Dumbbell Rectangular 120 3.10E+10 1.44E+10 130 4.06E+10 1.92E+10 140 3.61E+10 1.27E+10 144 1.E+ll - >. .*.'.‘ v, . 4:) 3.10E+10 4°06E+10 3.61E+10 a: "A o . S E 'T a: i” S —“-’ I a E . a .. - = I.E+10- 1.44E+10 L92E+10 1.27E+10 Q) U 0 Dumbbell I Rectangular 1.E+09 I I I I I I 115 120 125 130 135 140 145 Temperatures (“C) Figure 6.15 Comparison of cell population density between thinner dumbbell and thicker rectangular shapes at different foaming temperatures at 10 sec foaming time. Effect of thickness on average cell size of foams Average cell sizes, as shown in Table 6.12 and Table 6.16 were larger at higher temperatures, as discussed before. Average cell sizes of both samples linearly increased with foaming temperature. The cell size in the thinner dumbbell shape was slightly larger than in the thicker rectangular shape (4.40 pm). A 140°C foaming temperature, the average cell sizes of both dumbbell and rectangular shapes were almost the same (reached the largest cell size). 145 Table 6.12 Comparison of average cell size between thinner dumbbell and thicker rectangular shapes at different foaming temperatures at 10 sec foaming time. Temperature Average cell size (um) (°C) Dumbbell Rectaggular 120 4.98 4.40 130 5.67 5.06 140 6.62 6.64 7 1 A E a V .24 6‘ W E 1:) 3’3 498 a . i... a 5 J g / ODumbbell a) / .= I- / I Rectangular I / 4.40 4 T I' I r I I 115 120 125 130 I35 140 145 Temperatures (°C) Figure 6.16 Comparison of average cell size between thinner dumbbell and thicker rectangular shapes at the different foaming temperatures at 10 sec foaming time. 146 The aging effect on tensile properties Materials and methods The unfoamed and foamed Biomax® samples (foaming temperatures of 120 and 130°C, at 10 sec foaming time) were kept for 1 year at ambient conditions before testing. The unfoamed and foamed Biomax® samples were conditioned at 23°C and 50 % RH for 48 hours before testing. This storage condition is lower than the Tg of Biomax® so physical aging can occur. Results were compared to those at 2 weeks. Results and discussion The melting temperature results of unfoamed Biomax® and foamed samples showed no significant change between 2 weeks and 1 year as shown in Table 6.13. Table 6.13 The melting temperature results of unfoamed and foamed Biomax® at 2 weeks and at lyear storage time after foaming. Samples T92 (°C) % Decreased 2 weeks 1 year Unfoamed Biomax® 199.85i2.94 199233033 0.31 Fomaed 120°C 10 sec 199.58:l:0.27 199.1410.45 0.22 Fomaed 130°C 10 sec 199.80i1.77 197.99i0.79 0.90 The heat of fusion (AHf) results of unfoamed and foamed Biomax® decreased from 2 weeks to 1 year. This showed that the percent crystallinity of unfoamed and foamed Biomax® decreased after 1 year. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A15, showed only the 147 difference in heat of fusion between 2 weeks and 1 year at foaming temperature 130°C and foaming time 10 see was not significant. Table 6.14 The heat of fusion results of unfoamed Biomax® and foams at 2 weeks and at 1 year storage time after foaming. Samples AHr (J/g) % Decreased 2 weeks 1 year Unfoamed Biomax® 26.59i0.23 197221.99 25.81 F omaed 120°C 10 sec 29.71:l:1.42 23.01t0.27 22.54 Fomaed 130°C 10 sec 27.584009 22.62zl:l.87 17.97 Mechanical properties (tensile strength, percent elongation at break, and tensile modulus) were compared at lifetimes of 2 weeks and 1 year. Biomax® foams were brittle in tension, as shown in Figure 6.17. There was little or no macroscopic yielding prior to fracture, due to the inherent lack of tensile ductility. The results for tensile strength, as shown in Table 6.15, showed improvement in all foamed samples in tensile strength at 1 year. As can be seen in Table 6.17, tensile modulus of foamed samples (foaming time 120 and 130°C at foaming time 10 sec) increased, but of unfoamed Biomax® decreased. The unfoamed. Biomax® samples were observed to have absorbed water and curved, making the samples softer, very different from the foamed samples that were stiffer. This may be due to the CO2 gas which acted as plasticizer diffusing out from the foamed cells. The foamed samples at 1 year had higher stiffness and reduced percent elongation at break, as shown in Table 6.16. This also can be explained by the cohesional entanglement theory [8] that the cohesional entanglement will form by the parallel packing of the local neighboring of the polymer chains because of the interchain cohesion during physical aging. At a given temperature (120 and 130°C foaming temperature), with increasing 148 physical aging time, the density of cohesional entanglement increases and the parallel packing of the local neighboring segments, which the cohesional entanglements form, tends to be stronger [8]. The behaviors of physical aging of unfoamed and foamed Biomax® are complicated and further study is needed. The statistical analysis using one way ANOVA with 95% confidence interval, as shown in Appendix A, Table A16, showed only the difference in tensile strength between 2 weeks and 1 year at foaming temperature 130°C and foaming time 10 see was significant. For elongation at break, as shown in Appendix A, Table A17, indicated only the difference in percent elongation at break between 2 weeks and 1 year for Biomax® was significant. As seen in Appendix A, Table A18, there were no significant effects on tensile modulus. 40~ A a E ...._ _‘ e. V ‘ _- m 7) t H W - + -120C - "- '120C 1 year - -A- -|30c —-)(— 130 C 1 year + unfoamed Biomax —"— Unfoamed Biomax 1 year 100 150 Strain (%) Figure 6.17 Stress-strain curves of unfoamed Biomax® and foams between 2 weeks and lyear storage time. 149 Table 6.15 Tensile strength of unfoamed Biomax® and foams at 2 weeks and at 1 year storage time after foaming Materials Tensile Strength (MPQ % Increase 2 weeks 1 year Unfoamed Biomax® 360021.92 37.802289 5.00 F oamed 120 °C 10 sec 11.10:l:0.55 14.19:l:2.70 34.23 Foamed 130 °C 10 sec 9.02i0.56 22.822317 152.99 Table 6.16 Percent elongation at break of unfoamed Biomax® and foams at 2 weeks and at lyear storage time after foaming. Materials Elongation at break (%) % Increase 2 weeks 1 year Unfoamed Biomax:Q6 1203.64213224 9675025243 -19.58 Foamed 120 °C 10 sec 83.94zt19.93 37.08521 -55.95 Foamed 130 °C 10 sec 66.19:|:4.65 36.97:I:8.00 -43.94 Table 6.17 Tensile modulus of unfoamed Biomax® and foams at 2 weeks and at 1 year storage time after foaming Materials Tensile modulus (MPa) % Increase 2 weeks 1 year Unfoamed Biomaxr 6.1520.56 4.3020.60 -30.08 Foamed 120 °C 10 sec 0.832018 1.12:1:0.14 34.94 Foamed 130 °C 10 sec 05520.05 1.772036 221.82 150 Conclusions The foaming temperature and time affected all dimensions of the foamed samples, increasing with the foaming temperature and time, especially in thickness. Density also decreased and average cell size increased. The thickness of the samples affected the uniformity of the foams, as the inside of the foam had lower temperature than the outside surface during the nucleation process (hot glycerol bath). At the same conditions, the thinner foam had lower density, thinner cell walls, and larger cells. The rectangular shape (thicker) foams had higher density than the dumbbell shape at the same conditions, due to increased thickness of the samples. The statistical results showed both foaming temperature and time significantly affected foamed densities. The effect of aging on tensile strength, percent elongation at break, and tensile modulus was studied. The foamed samples at 1 year storage time appeared to be stiffer compared to the 2 week samples. Tensile strength and tensile modulus appeared to improve during this storage time and percent elongation at break to be reduced. However, the only statistically significant difference was for tensile strength at 130°C foaming temperature. 151 References l. Rachtanapun, P., Microcellular Foam of Polymer Blends of HDPE/PP and Their Composites with Wood Fiber, Ph.D. Dissertation, School of Packaging, Michigan State (2003). Doroudiani, S., Park, C. B., and Kortschot, M. T., J. Appl. Polym. Sci., 90, p 1421-1426 (2003). Doroudiani, S., Park, C. B., and Kortschot, M. T., J. Appl. Polym. Sci., 90, p 1412-1420, (2003). . Park, C. P., In Polymeric Foams and Foam Technology, Klempner D., Sendijarevic V., (ed), Hanser Gardener Publications, Cincinnati, Ohio, USA, p 267 (2004). Zhang, J. F. and Sun, X., Biodegradable Polymers for Industrial Applications, Ray, S. (ed), CRC Press LLC, Florida, p 275-276 (2005). Suh, K. W., In Polymeric Foams and Foam Technology, Klempner D., Sendijarevic V., (ed), Hanser Gardener Publications, Cincinnati, Ohio, USA, p 207 (2004). Kondo, H., Tanaka, T., Masuda, T., and Nagajima, A., Pure & Appl. Chem, 64 (12), p 1945-1952 (1992). Liao, K., Quan, D., and Lu, 2., European Polymer Journal, 38, p 157-162 (2002). 152 Chapter 7 Conclusions and Future Work Conclusions ® In this study, Biomax microcellular foams were successfully developed, yielding closed cell foams. Biomax® microcellular foams changed the translucent and brownish unfoamed Biomax® sample to white and opaque, with a smooth surface. The effects of the foaming process (foaming temperature and time) on size, density, cell population density, average cell size, morphology, tensile strength, percent elongation at break, and tensile modulus were studied for the batch process using C02 as a blowing agent at two days saturation time under 850 psi saturation pressure. The experimental results for tensile strength and tensile modulus were compared with the simple rule of mixtures, and with Moore’s empirical square power law, respectively. One way ANOVA with 95% confidence interval was used to analyze data. The aging effect on mechanical properties of foams was also studied. Based on this study, the conclusions can be summarized as: 1. Biomax® microcellular foams were successfully developed using the mini twin-screw extruder with injection molding to prepare the samples and using a batch ® process with C02 as a blowing agent. Biomax samples changed from translucent and brownish to white and opaque, with smooth surfaces when they were foams. 2. The saturation behavior of Biomax® samples was studied under 850 psi C02 in a chamber from 1 day to 6 days. CO; uptake of two days was selected for the experiment, resulting in about 7.1 1% weight gain. 153 3. The effects of the foaming temperature and foaming time on size, density, average cell size, cell population density, tensile strength, percent elongation at break, and tensile modulus of Biomax® microcellular foam were studied. The results were analyzed using one way ANOVA with a 95% confidence interval. As the foaming temperature increased, polymer chain stiffness decreased, causing the viscosity and surface tension of polymer to decrease, and resulting in faster diffusion rate of CO2 gas from the polymer to the cells, causing cell expansion to increase, size of foam to increase, larger cell sizes, the cell wall thickness to decrease and density to decrease. With increased foaming time, CO2 gas had a longer time to diffuse into the cells, so the cell expanded more, leading to a larger size, reduced density and thinner walls. 4. Tensile strength and tensile modulus also decreased with higher foaming temperature and foaming time. Experimental results for tensile strength and tensile modulus at different foaming temperatures showed a good fit with the model, and tensile modulus at different foaming times showed a good fit with the model. 5. The effect of aging effect on tensile strength, percent elongation at break, and tensile modulus of microcellular foams was studied. After one year, foamed samples appeared to become slightly stiffer. Tensile strength appeared to increase slightly, but percent elongation to decrease. 6. The thickness of the samples affected density, morphology, average cell size, and the uniformity of the foams. At higher thicknesses, the inside of the foam needed a longer time for gas to diffuse and higher foaming temperature because of the increased temperature gradient. The outside surface of foams had larger cell sizes than the inside of 154 the samples. At the same processing conditions, the thicker samples had higher density, smaller cell sizes, and thicker cell walls than did the thinner samples. 7. A variety of cell sizes with the same density can be made by carefully controlling the processing conditions: foaming temperature and foaming time. Similarily, a variety of densities can be developed with the same cell size. 155 Future work Biomax® microcellular foam has the potential to replace some uses of conventional foams due to its environmental advantages. Biomax® microcellular foams can be developed and used for some applications such as fast food containers, packaging; cushions, medical applications such as time-release drugs, etc. In this research, Biomax® microcellular foams were developed and studied, additional study to improve the foaming process and foam properties. Here are some ideas for future work on Biomax ® microcellular foams: 1. ® Biomax microcellular foam can be developed in a continuous process for commercial production. The effects of processing conditions on compression strength, and thermal property of Biomax® microcellular foams should be studied to evaluate potential application in structural foams. The bending and curving problems of foamed samples during the foaming process in the hot glycerol bath limited the range of foaming temperatures used. Excessive time was required for the saturation process, and the small CO2 chamber limited the sample sizes, so systems of foaming such as plate compression molding or a semi-continuous process can be designed for research purposes. Blending with other biodegradable polymers such as modified starch to reduce the material cost, or nano-clay to improve mechanical properties or barrier properties, can be investigated. 156 5. Degradability of Biomax® microcellular foams should be evaluated in a composting facility and compared with neat Biomax®. 157 Appendix A Table Al One way ANOVA results: density with different foaming temperatures. Sum of Mean Squares df Square F P value Between 3.438 5 .688 382.052 .000 Groups Within Groups .027 15 .002 Total 3.465 20 Multiple Comparisons 95% Confidence Interval Mean Std. P Lower Upper (I) type (J) type Difference (I-J) Error value. Bound Bound Biomax 100 c 10 s .35600(*) .03550 .000 .2208 .4912 110 c 10 s .70600(*) .03098 .000 .5880 .8240 120 c 10 s .82400(*) .02683 .000 .7218 .9262 130 c 10 s I.01600(*) .03098 .000 .8980 1.1340 140 c 10 s l.09600(*) .03098 .000 .9780 1.2140 100 c 10 s Biomax -.35600(*) .03550 .000 -.4912 -.2208 110 c 10 s .35000(*) .03873 .000 .2025 .4975 120 c 10 s .46800(*) .03550 .000 .3328 .6032 130 c 10 s .66000(*) .03873 .000 .5125 .8075 140 c 10 s .74000(*) .03873 .000 .5925 .8875 l '10 c 10 s Biomax -.70600(*) .03098 .000 -.8240 -.5880 100 c 10 s -.35000(*) .03873 .000 -.4975 -.2025 120 c 10 s .1 1800 .03098 .050 .0000 .2360 130 c 10 s .31000(*) .03464 .000 .1781 .4419 140 c 10 s .39000(*) .03464 .000 .2581 .5219 120 c 10 s Biomax -.82400(*) .02683 .000 -.9262 -.7218 100 c 10 s -.46800(*) .03550 .000 -.6032 -.3328 110 c 10 s -.l 1800 .03098 .050 -.2360 .0000 130 c 10 s .l9200(*) .03098 .001 .0740 .3100 140 c 10 s .27200(*) .03098 .000 .1540 .3900 130 c 10 s Biomax -1 .01600(*) .03098 .000 -l.l340 -.8980 100 c 10 s -.66000(*) .03873 .000 -.8075 -.5125 110 c 10 s -.31000(*) .03464 .000 -.4419 -.l781 120 c 10 s -.l9200C") .03098 .001 -.3100 -.0740 140 c 10 s .08000 .03464 .417 -.0519 .2119 140 c 10 s Biomax -1 .09600(*) .03098 .000 -l .2140 -.9780 100 c 10 s -.74000(*) .03873 .000 -.8875 -.5925 110 c 10 s -.39000(*) .03464 .000 -.5219 -.2581 120 c 10 s -.27200(*) .03098 .000 -.3900 -.1540 130 c 10 s -.08000 .03464 .417 -.2119 .0519 * The mean difference is significant at the .05 level 158 Table A2 One way ANOVA results: tensile strength with different foaming temperatures. Sum of Mean Squares df Square F P value Between 741.228 4 185.307 125.113 .000 Groups Within Groups 37.028 25 1.481 Total 778.256 29 Multiple Comparisons Mean 95% Confidence Interval Difference P value (1) temp (J) temp (I-l) Std- EITOI’ Lower Bound Upper Bound 100 C 110 C 5.98850 .81640 .000 3.2765 8.7005 120 C 10.67341 .71058 .000 8.3129 13.0339 130 C 12.78450 .81640 .000 10.0725 15.4965 140 C 16.60450 .81640 .000 13.8925 19.3165 110 C 100 C -5.98850 .81640 .000 —8.7005 -3.2765 120 C 4.68491 .65641 .000 2.5044 6.8654 130 C 6.79600 .76970 .000 4.2391 9.3529 140 C 10.61600 .76970 .000 8.0591 13.1729 120 C 100 C -10.67341 .71058 .000 -13.0339 -8.3129 1 10 C -4.68491 .6564] .000 —6.8654 -2.5044 130 C 2.11109 .65641 .061 -.0694 4.2916 140 C 5.93109 .65641 .000 3.7506 8.1116 130 C 100 C 42.78450 .81640 .000 -15.4965 -10.0725 1 10 C -6.79600 .76970 .000 —9.3529 -4.2391 120 C -2.1 l 109 .65641 .061 —4.2916 .0694 140 C 3.82000 .76970 .001 1.2631 6.3769 140 C 100 C -16.60450 .81640 .000 —I9.3 165 —13.8925 110 C 40.61600 .76970 .000 -l3.l729 -8.0591 120 C -5.93109 .65641 .000 -8.1116 -3.7506 130 C -3.82000 .76970 .001 -6.3769 -1 .2631 159 Table A3 One way ANOVA results: relative tensile strength with relative density (foaming temperature). Sum of Mean Squares df Square F P value Between 2.490 5 .498 377.197 .000 Groups Within Groups .038 29 .001 Total 2.528 34 Correlations Relative Density Relative tensile (temp) strength (temp) Relative Density Pearson H (temp) Correlation l '978( ) Sig. (2-tailed) .000 N 35 35 Relatlve tens1le Pearson. .978 (H) 1 strength (temp) Correlat1on Sig. (2-tailed) .000 N 35 35 H Correlation is significant at the 0.01 level (2-tailed). 160 Table A4 One way ANOVA results: percent elongation at break with different foaming temperatures. Sum of Squares df Mean Square F P value Between 5297301936 5 1059460387 316.130 .000 Groups Within Groups 73729.684 22 3351.349 Total 5371031620 27 Multiple Comparisons Mean Difference l 95% Confidence Interval (l) temp (J) temp (H) Std- ETTOT P V81“?— Lower Bound Upper Bound 100c 110C 80.99200 38.83436 .985 472.6516 110.6676 120C -35.69275 40.93501 .977 485.0150 113.6295 130C 47.94000 38.83436 .999 459.5996 123.7196 140C 44.83580 38.83436 1.000 456.4954 126.8238 Biomax 4155.395400) 38.83436 .000 4297.0550 4013.7358 110C 100C 30.99200 38.83436 .985 -110.6676 172.6516 120C 4.70075 38.83436 1.000 446.3603 136.9588 130C 13.05200 36.61338 1.000 420.5059 146.6099 140C 16.15620 36.61338 .999 417.4017 149.7141 Biomax 4124403400) 36.61338 .000 42579613 -990.8455 120C 100C 35.69275 40.93501 .977 413.6295 185.0150 110C 4.70075 38.83436 1.000 436.9588 146.3603 130C 17.75275 38.83436 .999 423.9068 159.4123 140C 20.85695 38.83436 .998 420.8026 162.5165 Biomax 4119.702650) 38.83436 .000 4261.3622 -978.0431 l30C 100C 17.94000 38.83436 .999 423.7196 159.5996 110C 43.05200 36.61338 1.000 446.6099 120.5059 120C 47.75275 38.83436 .999 459.4123 123.9068 140C 3.10420 36.61338 1.000 430.4537 136.6621 Biomax 4137455400) 36.61338 .000 4271.0133 4003.8975 140C 100C 14.83580 38.83436 1.000 426.8238 156.4954 110C 46.15620 36.61338 .999 449.7141 117.4017 120C -20.85695 38.83436 .998 462.5165 120.8026 130C 4.10420 36.61338 1.000 436.6621 130.4537 Biomax 4140559600) 36.61338 .000 4274.1175 4007.0017 Biomax 100C 1155395400) 38.83436 .000 1013.7358 1297.0550 110C 1124403400) 36.61338 .000 990.8455 1257.9613 120C 1119.702650) 38.83436 .000 978.0431 1261.3622 130C 1137455400) 36.61338 .000 1003.8975 1271.0133 140C 1140559600) 36.61338 .000 1007.0017 1274.1175 * The mean difference is significant at the .05 level. 161 Table A5 One way ANOVA results: tensile modulus with different foaming temperatures. Sum of Mean Squares df Square F P value Between 92.475 5 18.495 305.294 .000 Groups Within Groups 1.333 22 .061 Total 93.807 27 Multiple Comparisons (J) Mean 7 95% Confidence Interval (I) temp temp Difference (I-J) Std. Error P value Lower Bound Upper Bound 100.00 110.00 .91750(*) .17404 .002 .2826 1.5524 120.00 1.39650(*) .1651 1 .000 .7942 1.9988 130.00 1.67050(*) .16511 .000 1.0682 2.2728 140.00 1.82250(*) .165 '11 .000 1.2202 2.4248 Biomax -3.27750(*) .16511 .000 -3.8798 -2.6752 1 10.00 100.00 -.91750(*) .17404 .002 -l .5524 -.2826 120.00 .47900 .16511 .180 -.1233 1.0813 130.00 .75300(*) .16511 .008 .1507 1.3553 140.00 .90500(*) .1651 1 .001 .3027 1.5073 Biomax -4.19500(*) .16511 .000 -4.7973 -3.5927 120.00 100.00 -1.39650(*) .16511 .000 -1.9988 -.7942 110.00 -.47900 .16511 .180 -l .0813 .1233 130.00 .27400 .15567 .686 -.2938 .8418 140.00 .42600 .15567 .231 -.l418 .9938 Biomax -4.67400(*) .15567 .000 ~5.2418 -4. 1062 130.00 100.00 -1.67050(*) .16511 .000 -2.2728 -1.0682 110.00 -.75300(*) .16511 .008 -1.3553 -.1507 120.00 -.27400 .15567 .686 -.8418 .2938 140.00 .15200 .15567 .963 -.4158 .7198 Biomax -4.94800(*) .15567 .000 —5.5158 -4.3802 140.00 100.00 -1 .82250(*) .16511 .000 -2.4248 -1.2202 110.00 -.90500(*) .16511 .001 -1.5073 -.3027 120.00 -.42600 .15567 .231 -.9938 .1418 130.00 -.15200 .15567 .963 -.7198 .4158 Biomax -5.10000(*) .15567 .000 -5.6678 -4.5322 Biomax 100.00 3.27750(*) .16511 .000 2.6752 3.8798 110.00 4.195000) .16511 .000 3.5927 4.7973 120.00 4.67400(*) .15567 .000 4.1062 5.2418 130.00 4.94800(*) .15567 .000 4.3802 5.5158 140.00 5.10000(*) .15567 .000 4.5322 5.6678 * The mean difference is significant at the .05 level. 162 Table A6 One way ANOVA results: relative tensile modulus with relative density (foaming temperature) Sum of Mean Squares df Square F Sig. Between 2.976 5 .595 303.182 .000 Groups Within Groups .041 21 .002 Total 3.017 26 Correlations Relative Relative Tensile Density(Temp) Modulus (Temp) Relative Pearson H Density(Temp) Correlation 1 '981( ) Sig. (2-tailed) .000 N 27 27 Relative Tensile Pearson 981 (H) 1 Modulus (Temp) Correlation ' Sig. (2—tailed) .000 N 27 27 ** Correlation is significant at the 0.01 level (2-tailed). 163 Table A7 One way ANOVA results: density with different foaming times. Sum of Mean Squares df Square F P value Between 3.587 3 1.196 986.658 .000 Groups Within Groups 0.019 16 0.001 Total 3.607 19 Multiple Comparisons Mean 95% Confidence Interval Difference P value (1) Time (J) Time (I-J) Std- Error Lower Bound Upper Bound 10 Second 20 Second 0.18062* 0.2202 .000 .1120 .2493 30 second 0.22436* . 0.2202 .000 .1557 .2930 Biomax -0.82358* 0.2202 .000 -.8922 -.7549 20 Second 10 Second -0.l8062* 0.2202 .000 -.2493 -.1120 30 second 0.04374 . 0.2202 .304 -.0249 .1 124 Biomax -1.00420* 0.2202 .000 -1.0728 -.9356 30 second 10 Second -0.22436* 0.2202 .000 -.2930 -.1557 20 Second -0.04374 . 0.2202 .304 -.1 124 .0249 Biomax -1.04794* 0.2202 .000 -1.1 166 -.9793 Biomax 10 Second 0.82358* 0.2202 .000 .7549 .8922 20 Second 1.00420* . 0.2202 .000 .9356 1.0728 30 second 1.04794* 0.2202 .000 .9793 1.1166 * The mean difference is significant at the .05 level. 164 Table A8 One way ANOVA results: tensile with different foaming times. Sum of Mean Squares df Square F P value Between 39.028 2 19.514 40.121 .000 Grows Within Groups 8.268 17 .486 Total 47.297 19 Multiple Comparisons Mean ‘ l 95% Confidence Interval Difference Std. P value Lower Upper (1) Time (J) Time (I-J) Error Bound Bound 10 Second 20 Second 1.10909 .40720 .046 .0177 2.2004 30 second 3.36909 .37616 .000 2.3609 4.3772 20 Second 10 Second -1.10909 .40720 .046 -2.2004 -.0177 30 second 2.26000 .46784 .001 1.0061 3.5139 30 second 10 Second -3.36909 .37616 .000 -4.3772 -2.3609 20 Second -2.26000 .46784 .001 -3.5139 -1.0061 165 Table A9 One way ANOVA results: (foaming time). ** relative tensile strength with relative density Sum of Mean Squares df Square F P value gems“ 2.515 3 .838 864.982 .000 roups Within Groups .020 21 .001 Total 2.536 24 Correlations Relative Density Relative Tensile (time) Strength (time) Relative Density Pearson H (timg Correlation 1 '984( ) Sig. (2-tailed) .000 N 25 25 Relative Tensile Pearson H Strength (time) Correlation '984( ) l Sig. (2-tai1ed) .000 N 25 25 Correlation is significant at the 0.01 level (2-tailed). 166 Table A10 One way ANOVA results: percent elongation at break with different foaming times. Sum of Squares df Mean Square F P value Between 4615995166 3 1538665055 279.060 .000 Groups Within Groups 71678.612 13 5513.739 Total 4687673779 16 Multiple Comparisons 95% Confidence Interval Mean Difference Lower (1) time (J) time (I-J) Std. Error P value Bound Upper Bound 108 208 13.35375 56.71286 .996 -168.0529 194.7604 308 47.76395 49.81147 .820 -111.5674 207.0953 Biomax -l l l9.70265(*) 49.81147 .000 -1279.0340 -960.3713 208 108 -13.35375 56.71286 .996 -194.7604 168.0529 308 34.41020 54.22786 .938 -139.0478 207.8682 Biomax -1133.05640(*) 54.22786 .000 -1306.5144 -959.5984 308 [08 -47.76395 49.81147 .820 -207.0953 111.5674 208 -34.41020 54.22786 .938 -207.8682 139.0478 Biomax -1167.46660(*) 46.96271 .000 -1317.6856 -1017.2476 Biomax lOS 1119.70265(*) 49.81147 .000 960.3713 1279.0340 208 1133.05640(*) 54.22786 .000 959.5984 1306.5144 30$ 1167.46660(*) 46.96271 .000 1017.2476 1317.6856 * The mean difference is significant at the .05 level. 167 Table All One way ANOVA results: tensile modulus with different foaming time. Sum of Mean Squares df Square F P value Between 87.527 3 29.176 454.773 .000 Groups Within Groups .962 15 .064 Total 88.489 18 Multiple Comparisons Mean Std. 95% Confidence Interval (1) time (.1) time Difference (I-J) Error P value Lower Bound Upper Bound 10.00 20.00 .21850 .16991 .655 -.3151 .7521 30.00 .36000 .16019 .213 -.1431 .8631 Biomax 4.67400(*) .16019 .000 -5.1 771 -4.1709 20.00 10.00 -.21850 .16991 .655 -.7521 .3151 30.00 .14150 .16991 .873 -.3921 .6751 Biomax -4.89250(*) .1699] .000 -5.4261 -4.3589 30.00 10.00 -.36000 .16019 .213 -.8631 .1431 20.00 -.14150 .16991 .873 -.6751 .3921 Biomax -5.03400(*) .16019 .000 -5.5371 4.5309 Biomax 10.00 4.67400(*) .16019 .000 4.1709 5.1771 20.00 4.89250(*) .16991 .000 4.3589 5.4261 30.00 5.03400(*) .16019 .000 4.5309 5.5371 * The mean difference is significant at the .05 level. 168 Table A12 One way ANOVA results: relative tensile modulus with relative density (foaming time). Sum of Mean Squares df Square F P value Between 2.851 3 .950 512.684 .000 Groups Within Groups .028 15 .002 Total 2.878 18 Correlations Relative Relative Tensile Density (time) Modulus (Time) Relative Density Pearson *4 (time) Correlation l '995( ) Sig. (2-tailed) .000 N 19 19 Relative Tensile Pearson 995C") 1 Modulus (Time) Correlation ' Sig. (2-tailed) .000 N 19 19 ** Correlation is significant at the 0.01 level (2-tailed). 169 Table A13 UNIANOVA results: density with foaming temperature and foaming time (rectangular shape). Type III Sum of Mean Source Sgpares df Square F P value CREE? 2.913(3) 7 .416 141.317 .000 Intercept 31.068 1 31.068 10551.369 .000 temp .934 3 .311 105.700 .000 time 1.069 2 .535 181.539 .000 temp * time .005 1 .005 1.824 .184 Error .133 45 .003 Total 37.093 53 Corrected Total 3.045 52 a R Squared = .956 (Adjusted R Squared = .950) Multiple Comparisons: temperature Mean 95% Confidence Interval Difference (I) temp (J) temp (I—J) Std- Error P value Lower Bound Upper Bound Biomax 120.00 5836(*) .02827 .000 .4928 .6744 130.00 4896(*) .02827 .000 .3988 .5804 140.00 6652(*) .02855 .000 .5735 .7569 150.00 6650(*) .03177 .000 .5629 .7670 120.00 Biomax - 5836(*) .02827 .000 - 6744 -.4928 130.00 - 0939(*) .02051 .001 - 1598 - 0281 140.00 0817(*) .02090 .009 .0145 1488 150.00 0814(*) .02512 .047 .0007 1621 130.00 Biomax - 4896(*) .02827 .000 -.5804 - 3988 120.00 0939(*) .02051 .001 .0281 .1598 140.00 1756(*) .02090 .000 1085 .2427 150.00 1753(*) .02512 .000 .0947 2560 140.00 Biomax - 6652(*) .02855 .000 - 7569 - 5735 120.00 - 0817(*) .02090 .009 - 1488 — 0145 130.00 - l756(*) .02090 .000 -.2427 - 1085 150.00 - 0003 .02544 1.000 - 0820 .0814 150.00 Biomax - 6650(*) .03177 .000 - 7670 - 5629 120.00 - 0814(*) .02512 .047 - 1621 -.0007 130.00 - 1753(*) .02512 .000 -.2560 -.0947 140.00 .0003 .02544 1.000 - 0814 .0820 Based on observed means. ° The mean difference is significant at the .05 level. 170 Multiple Comparisons: time Mean l 95% Confidence Interval Difference Std. Lower Upper (1) time (J) time (I-J) Error P value Bound Bound Biomax 5.00 .4869(*) .02713 .000 .4081 .5657 10.00 .6449(*) .02700 .000 .5664 .7233 15.00 .7212(*) .03177 .000 .6289 .8134 5.00 Biomax -.4869(*) .02713 .000 -.5657 -.4081 10.00 .1580(*) .01695 .000 .1087 .2072 15.00 .2343(*) .02383 .000 .1651 .3035 10.00 Biomax -.6449(*) .02700 .000 -.7233 -.5664 5.00 -.1580(*) .01695 .000 -.2072 -.1087 15.00 .0763(*) .02368 .024 .0075 .1451 15.00 Biomax -.7212(*) .03177 .000 -.8l34 -.6289 5.00 -.2343(*) .02383 .000 -.3035 -.1651 10.00 -.0763(*) .02368 .024 -.1451 -.0075 Based on observed means. * The mean difference is significant at the .05 level. 171 Table A14 One way ANOVA results: density of dumbbell and rectangular shape with foaming temperature. Sum of Mean Squares df Square F P value Between 4.012 5 .802 240.029 .000 Groups Within Groups .087 26 .003 Total 4.099 31 Multiple Comparisons Mean 95% Confidence Interval Difference Std. P Lower Upper (11 type (J) type (1'1) EITOT value Bound Bound Biomax Dumbell 120C 10s Dumbell .82358(*) .03657 .000 .6921 .9551 130c 10 s Dumbell 1.01558(*) .04223 .000 .8637 1.1674 Biomax Retangular .00962 .03657 1.000 —.1219 .1411 120c 108 Rectangular .45640("‘) .03386 .000 .3346 .5782 130c 10 s Rectangular .68701(*) .03386 .000 .5652 .8088 120c lOs Dumbell Biomax Dumbell -,82358(*) .03657 .000 -.9551 -.6921 130c 10 s Dumbell .19200(*) .04223 .007 .0401 .3439 Biomax Retangular —.81396(*) .03657 .000 -.9455 -.6824 120c 105 Rectangular -.36717(*) .03386 .000 -.4889 —.2454 130c 10 s Rectangular -.13657(*) .03386 .021 -.2583 -.0148 1300 10 S Dumbell Biomax Dumbell -1.01558(*) .04223 .000 -1.l674 -.8637 120C lOs Dumbell -.19200(*) .04223 .007 —.3439 -.0401 Biomax Retangular -l.00596(*) .04223 .000 —l.1578 -.8541 120c 10$ Rectangular -.559l7(*) .03990 .000 -.7027 -.4157 130C 10 s Rectangular -.32857(*) .03990 .000 -.4721 -.1851 Biomax Retangular Biomax Dumbell -,00962 ,03657 1.000 -.l411 .1219 120c lOs Dumbell .81396(*) .03657 .000 .6824 .9455 130c 10 s Dumbell 1.00596(*) .04223 .000 .8541 1.1578 120C lOS Rectangular .44679(*) .03386 .000 .3250 .5685 130c 10 s Rectangular .67739(*) .03386 .000 .5556 .7992 Ligfahgilar B‘Oma" Dumbell -.45640(*) .03386 .000 -.5782 -.3346 120c lOs Dumbell .36717(*) .03386 .000 .2454 .4889 130c 10 s Dumbell .55917(*) .03990 .000 .4157 .7027 Biomax Retangular -.44679(*) .03386 .000 —.5685 -.3250 130c 10 S Rectangular .23060(*) .0309] .000 .1 195 .3418 11136253331614 B‘Oma" Dumbell -.68701(*) .03386 .000 -.8088 -.5652 120c 105 Dumbell .13657("‘) .03386 .021 .0148 .2583 130c 10 s Dumbell .32857(*) .03990 .000 .1851 .4721 Biomax Retangular -.67739('*) .03386 .000 —.7992 -.5556 120c 105 Rectangular -.23060('*) .03091 .000 -.3418 -.l 195 ° The mean difference is significant at the .05 level 172 Table A15 One way ANOVA results: aging effect on tensile strength with different foaming temperatures. 173 Multiple Comparisons Mean 95% Confidence Interval 1) type (.1) type Difference (1- Std. Error P value. Lower Upper J) Bound Bound 1 year Biomax 6.86167(*) 1.25905 .011 1.6085 12.1148 2 weeks 120C -3.12000 1.37922 .458 -8.8746 2.6346 ;;::: 1 year 120C 3.57500 1.25905 .251 -1.6782 8.8282 2 weeks 130C -.99500 1.37922 .989 -6.7496 4.7596 1 year 130C 3.96167 1.25905 .176 -l.2915 9.2148 2 weeks Biomax -6.86167(*) 1.25905 .011 -12.1148 -1.6085 2 weeks 120C -9.98167(*) 1.25905 .001 -15.2348 -4.7285 315:; 1 year 120C -3.28667 1.12613 .230 -7.9852 1.4119 2 weeks 130C -7.85667(*) 1.25905 .004 -13.1098 -2.6035 1 year 130C -2.90000 1.12613 .335 -7.5986 1.7986 2 weeks Biomax 3.12000 1.37922 .458 -2.6346 8.8746 1 year Biomax 9.98167(*) 1.25905 .001 4.7285 15.2348 2 weeks 120C 1 year 120C 6.695000) 1.25905 .012 1.4418 11.9482 2 weeks 130C 2.12500 1.37922 .787 -3.6296 7.8796 1 year 130C 7.08167(*) 1.25905 .009 1.8285 12.3348 2 weeks Biomax —3.57500 1.25905 .251 -8.8282 1.6782 1 year Biomax 3.28667 1.12613 .230 -1.4119 7.9852 1 year 120C 2 weeks 120C -6.69500(*) 1.25905 .012 -11.9482 4 .4418 2 weeks 130C —4.57000 1.25905 .098 -9.8232 .6832 1 year 130C .38667 1.12613 1.000 -4.3119 5.0852 2 weeks Biomax .99500 1.37922 .989 -4.7596 6.7496 1 year Biomax 7.85667("') 1.25905 .004 2.6035 13.1098 2 weeks 130C 2 weeks 120C .2.12500 1.37922 .787 -7.8796 3.6296 1 year 120C 4.57000 1.25905 .098 -.6832 9.8232 1 year 130C 4.95667 1.25905 .067 -.2965 10.2098 2 weeks Biomax —3.96167 1.25905 .176 -9.2148 1.2915 1 year Biomax 2.90000 1.12613 .335 —1.7986 7.5986 1 year 130C 2 weeks 120C -7.08167(*) 1.25905 .009 42.3348 4.8285 1 year 120C -.38667 1.12613 1.000 -5.0852 4.3119 1 2 weeks 130C -4.95667 1.25905 .067 40.2098 .2965 1 1 1* The mean difference is significant at the .05 level. *1 174 Table A16 One way ANOVA results: aging effect on tensile strength with different foaming temperatures. Sum of Mean Squares df Square F P value Between Groups 4588.176 5 917.635 259.660 .000 Within Groups 95.417 27 3.534 Total 4683.593 32 Multiple Comparisons 95% Confidence Mean Interval Difference Lower Upper (1) type (J) type (I-J) Std. Error P value Bound Bound Biomax 2weeks 120clOs 2weeks 25.43691(*) 1.01394 .000 21.8009 29.0729 130c108 2weeks 27.54800(*) 1.18895 .000 23.2844 31.8116 Biomax 1 year -1.25980 1.18895 .949 —5.5234 3.0038 120clOs 1 year 22.36925(*) 1.26107 .000 17.8471 26.8914 1300 10 s 1 year 13.70400(*) 1.37288 .000 8.7809 18.6271 120clOs 2weeks Biomax 2weeks -25.43691(*) 1.01394 .000 -29.0729 -21.8009 130c103 2weeks 2.11109 1.01394 .516 -1.5249 5.7471 Biomax 1 year -26.69671(*) 1.01394 .000 -30.3327 -23.0607 120clOs 1 year -3.06766 1.09762 .204 -7.0037 .8684 130c 10 s 1 year -11.7329l(*) 1.22444 .000 -l6.1238 —7.3420 l30clOs 2weeks Biomax 2weeks -27.54800(*) 1.18895 .000 -31.8116 -23.2844 120clOs 2weeks -2.11109 1.01394 .516 -5.7471 1.5249 Biomax 1 year —28.80780(*) 1.18895 .000 -33.0714 -24.5442 120clOs 1 year -5.l7875(*) 1.26107 .017 -9.7009 -.6566 l30c 10 s 1 year -l3.84400(*) 1.37288 .000 -18.7671 -8.9209 Biomax 1 year Biomax 2weeks 1.25980 1.18895 .949 -3.0038 5.5234 120clOs 2weeks 26.69671(*) 1.01394 .000 23.0607 30.3327 130c108 2weeks 28.80780(*) 1.18895 .000 24.5442 33.0714 120clOs 1 year 23.62905(*) 1.26107 .000 19.1069 28.1512 130c 10 s 1 year l4.96380(*) 1.37288 .000 10.0407 19.8869 1200105 1 year Biomax 2weeks -22.36925(*) 1.26107 .000 -26.8914 -l7.8471 120clOs 2weeks 3.06766 1.09762 .204 -.8684 7.0037 130c105 2weeks 5.17875(*) 1.26107 .017 .6566 9.7009 Biomax 1 year —23.62905(*) 1.26107 .000 -28.1512 -19.1069 130c 10 s 1 year -8.66525(*) 1.43579 .000 -13.8140 -3.5165 l30c 10 s 1 year Biomax 2weeks -13.70400(*) 1.37288 .000 -18.6271 -8.7809 120clOs 2weeks 11.73291(*) 1.22444 .000 7.3420 16.1238 l30clOs 2weeks 13.84400(*) 1.37288 .000 8.9209 18.7671 Biomax 1 year -l4.96380(*) 1.37288 .000 -19.8869 -10.0407 120clOs 1 year 8.66525(*) 1.43579 .000 3.5165 13.8140 * The mean difference is significant at the .05 level. 175 Table A17 One way ANOVA results: aging effect on percent elongation at break with different foaming temperatures. Sum of Squares df Mean Square F P value Between Groups 6645023615 5 1329004723 314.274 .000 Within Groups 84576.156 20 4228.808 Total 6729599770 25 Multiple Comparisons Mean P 95% Confidence Interval Difference value Lower Upper (1) type (1) type (1'1) Std- Error Bound Bound Biomax 2weeks 120c 108 2 weeks 1119.70265(*) 43.62297 .000 959.0987 1280.3066 130clOs 2weeks 1137.45540("‘) 41.12813 .000 986.0366 1288.8742 Biomax 1 year 236.14940(*) 41.12813 .001 84.7306 387.5682 120clOs 1 year 1165.94540(*) 47.49067 .000 991.1020 1340.7888 130c 105 1 year 1167.12040(*) 43.62297 .000 1006.5165 1327.7243 120c 10s 2 weeks Biomax 2weeks 4 1 19.70265(*) 43.62297 .000 42803066 -959.0987 130clOs 2weeks 17.75275 43.62297 .999 442.8512 178.3567 Biomax 1 year -883.55325(*) 43.62297 .000 4044.1572 -722.9493 120clOs 1 year 46.24275 49.66694 .969 436.6129 229.0984 130c 105 1 year 47.41775 45.98265 .953 421.8737 216.7092 130clOs 2weeks Biomax 2weeks -1137.45540(*) 41.12813 .000 42888742 -986.0366 120c 108 2 weeks 47.75275 43.62297 .999 478.3567 142.8512 Biomax 1 year —901.30600(*) 41.12813 .000 4052.7248 —749.8872 120clOs 1 year 28.49000 47.49067 .996 446.3534 203.3334 130c 10s 1 year 29.66500 43.62297 .992 430.9389 190.2689 Biomax 1 year Biomax 2weeks -236.14940(*) 41.12813 .001 -387.5682 -84.7306 120c 10s 2 weeks 883.55325(*) 43.62297 .000 722.9493 1044.1572 1300108 2weeks 901.30600(*) 41.12813 .000 749.8872 1052.7248 120clOs 1 year 929.79600(*) 47.49067 .000 754.9526 1104.6394 130c 105 1 year 930.97100(*) 43.62297 .000 770.3671 1091.5749 120clOs 1 year Biomax 2weeks -1165.94540(*) 47.49067 .000 4340.7888 -991.1020 120c 10$ 2 weeks -46.24275 49.66694 .969 -229.0984 136.6129 130clOs 2weeks -28.49000 47.49067 .996 -203.3334 146.3534 Biomax 1 year -929.79600(*) 47.49067 .000 -1 104.6394 —754.9526 130c 10$ 1 year 1.17500 49.66694 1.000 481.6806 184.0306 l30c 108 1 year Biomax 2weeks 4167.12040(*) 43.62297 .000 4327.7243 4006.5165 120c 105 2 weeks -47.41775 45.98265 .953 -216.7092 121.8737 130clOs 2weeks -29.66500 43.62297 .992 490.2689 130.9389 Biomax 1 year -930.97100(*) 43.62297 .000 40915749 -770.3671 120clOs 1 year 4.17500 49.66694 1.000 484.0306 181.6806 * The mean difference is significant at the .05 level. 176 Table A18 One way ANOVA results: aging effect on tensile modulus with different foaming temperatures. Sum of Mean Squares df Square F P value Between Groups 86.026 5 17.205 19.250 .000 Within Groups 18.770 21 .894 Total 104.796 26 Multiple Comparisons l 95% Confidence Mean ' Interval Difference P value Lower Upper (I) type (J) type (l-J) Std. Error Bound Bound Biomax 2 weeks Biomax 1 year .45333 .66850 .993 4.9960 2.9026 1200105 2 weeks 3.79800(*) .57247 .000 1.7005 5.8955 1200 10 s 1 year 3.80000(*) .61026 .000 1.5641 6.0359 1300105 2weeks 4.34500(*) .54583 .000 2.3452 6.3448 1300105 1 year 2.98333(*) .66850 .011 .5340 5.4326 Biomax 1 year Biomax 2 weeks -.45333 .66850 .993 -2.9026 1.9960 1200105 2 weeks 3.34467(*) .69043 .005 .8150 5.8743 1200 10 s 1 year 3.34667(*) .72206 .008 .7011 5.9922 1300105 2weeks 3.89167(*) .66850 .001 1.4424 6.3410 1300105 1 year 2.53000 .77192 .099 -.2982 5.3582 1200105 2 weeks Biomax 2 weeks -3.79800(*) .57247 .000 -5.8955 4.7005 Biomax 1 year —3.34467(*) .69043 .005 -5.8743 -.8150 1200 10 s 1 year .00200 .63420 1.000 -2.3216 2.3256 1300105 2weeks .54700 .57247 .966 4 .5505 2.6445 1300105 1 year -.81467 .69043 .920 -3.3443 1.7150 1200 10 s 1 year Biomax 2 weeks -3.80000(*) .61026 .000 -6.0359 4.5641 Biomax 1 year -3.34667(*) .72206 .008 -5.9922 -.7011 1200105 2 weeks -.00200 .63420 1.000 —2.3256 2.3216 1300105 2weeks .54500 .61026 .975 4.6909 2.7809 1300105 1 year -.81667 .72206 .932 —3.4622 1.8289 1300105 2weeks Biomax 2 weeks ~4.34500(*) .54583 .000 -6.3448 -2.3452 Biomax 1 year —3.89l67(*) .66850 .001 -6.3410 4.4424 1200105 2 weeks -.54700 .57247 .966 -2.6445 1.5505 1200 10 5 1 year -.54500 .61026 .975 -2.7809 1.6909 1300105 1 year 4.36167 .66850 .543 -3.8110 1.0876 1300105 1 year Biomax 2 weeks -2.98333(*) .66850 .011 —5.4326 -.5340 Biomax 1 year -2.53000 .77192 .099 -5.3582 .2982 1200105 2 weeks .81467 .69043 .920 4.7150 3.3443 1200 10 s lyear .81667 .72206 .932 4.8289 3.4622 1300105 2weeks 1.36167 .66850 .543 4.0876 3.8110 * The mean difference is significant at the .05 level 177 11111111111111)1111111111111